Xen is a fictional world that houses numerous alien species in an alternate dimension set in Valve's Half-Life series. Note that these aliens are not originally from Xen, but use it as a border world to access other worlds in other dimensions, such as Earth.
Bullsquids are unintelligent, bipedal, highly aggressive creatures that appear throughout the Black Mesa Research Facility due to a dimensional rift. Bullsquids appear to be able to survive, if not thrive, in environments that are unfriendly or even toxic to humans, including sewers and pools of radioactive, chemical, or biological waste. In Xen, Bullsquids are sometimes found near pools of an unknown liquid that has healing properties to humans.
The body of a Bullsquid is vaguely similar in appearance to that of a small theropod, with two short, muscular legs and a thick tail that tapers to a point. The thorax abruptly joins the creature's head with no neck or other visible separation. A typical specimen stands roughly one metre tall and about two metres in length. Two slit-pupiled eyes are mounted on either side of its head.
The Bullsquid's most striking feature, however, is the collection of bright red, tentacular protrusions which surround its mouth and allow it to grasp prey. Usually a Bullsquid will only attack by spitting globules of acid at enemies, but when in close range, the Bullsquid can also charge an opponent with surprising speed and force. The Bullsquid's skin is a sandy brown covered on its upper surfaces with dark spots. The tail features a pair of prominent, outlined spots, one on either side, that are noticeably different than the dark spots on its back. Bullsquids display a very territorial disposition, as they have often been seen attacking other creatures and even members of their own species. They viciously attacked Headcrabs in Half-Life, usually not stopping until all Headcrabs in the vicinity had been eliminated.
The bullsquid has an array of offensive capabilities: At close range, a hostile bullsquid will either maul its victim with its teeth, or suddenly spin around, delivering a powerful strike with its tail, often causing a gibbing. Additionally, the bullsquid is able to "spit" a toxic, bile-colored substance from its mouth. While not overly accurate or fast, it causes moderate damage, even at long range.
The Houndeye, or Sound Dog, is one of the organisms which appears in Black Mesa following the opening of a portal to Xen, and has an appropriately alien appearance. Possessing three legs and only a thorax, the Houndeye is a faint yellow-green in color, with electric blue tiger-like stripes adorning the spine of the organism. In place of a head there is a large, black eye that is protected by sideways-blinking eyelids. They apparently communicate through a series of high-pitched sounds somewhat similar to the barking of dogs.
The Houndeye is an example of a social animal in terms of pack hunting. While a shy and timid organism by itself, groups of three or more creatures display resonant behavior, emitting destructive harmonics capable of injuring those they attack. This behavior came about through Valve's original plan of Houndeyes being neutral or even friendly towards the player, until they realized playtesters shot them down regardless and dropped the idea.
Although they are not seen displaying this trait often, Houndeyes are omnivorous creatures, as one is seen eating a dead security guard in Half-Life: Blue Shift. A Houndeye's mouth is located on the underside of its belly, close to the eye, filled with what appears to be fang-like incisors.
Barnacles appear in Half-Life and Half-Life 2, and are also usable as a cross between a weapon and a biological grappling hook in Opposing Force. They are predators that attach to ceilings, the undersides of bridges, or similar areas and drop down a very long, highly-adhesive tongue. When something gets caught on a barnacle's tongue, it retracts it into its maw and consumes the prey, frequently dismembering it while it is still alive. Anything not consumed is dropped. Barnacles are known to eat almost anything that's alive: all varieties of headcrab, allied rebels, Combine soldiers, zombies, and antlions. Although barnacles will eat poisonous headcrabs, they die immediately after ingestion. If consuming zombies, a barnacle will eat only the headcrab and reject the human body attached to it. If an object is not edible, they drop it and await new prey. Barnacles will "remember" the last inedible object they are "fed" with and will not retract their tongue in an attempt to eat the same object if it is presented to them a second time. When a barnacle is killed, it will disgorge chunks of recent victims, after which its inner mouth will hang loosely from its body.
Tentacles are creatures (reminiscent of Star Wars' sarlacc) that can smash and grab victims with large, pointed heads. Their bodies are divided into segments reminiscent of some types of bamboo. Despite possessing what looks like an eye, they appear to be blind, instead responding to sound and vibrations.
Tentacles are found emerging from pits and may retreat into these when damaged. This organism is first encountered as a group of three within a chamber for testing rocket engines. The impression is that there is a single organism with three tentacles which is later confirmed by the sighting of a squid-like body with three stumps when the player travels through the area below after defeating the tentacles. However, single tentacles are seen later in the game.
The Gargantua, nicknamed "Garg", is a gigantic upright creature that can crush opponents with fire and sheer force. It is mostly blue in color, has a crocodile-like head, a thick back plating which resembles an armadillo, and has one yellow eye that glows red when it perceives a hostile unit. In place of hands, it has two claw like appendages which can be opened to emit jets of intense heat. These claw weapons are designed to open vertically, not horizontally. Though the flamethrower weapon is short-ranged, it is still effective, known to blow up machinery, as in Half-Life: Uplink. Gargantuas can make loud roars that can damage structures. An example of the loud roars is in Uplink, near the ending sequence before the player gets trapped, the creature's loud roar makes a vent that the player is crawling through collapse. It can also send a shock wave towards its opponents by powerfully stomping the ground. This shock wave, which appears as a red dot that shoots red sparks behind it, increases in speed as it travels and can traverse vertical structures. The Gargantua moves fast and is very strong: it can smash or toss large obstacles from its path with ease. Gargantuas are very determined pursuers, and wait patiently for their target to come out of an area they cannot reach for a considerable amount of time before moving on to do something else.
Gargantuas can be found on Xen, because they traveled through the portals opened in Black Mesa. They share some physical characteristics with Vortigaunts, such as their backwards-jointed legs, hoofed feet, and vestigial arms jutting from their abdomen. However, each Vortigaunt has one vestigial arm while the Gargantua has two. Gargantuas can be seen in Half-Life in the chapters "Power Up", "Surface Tension", "Interloper", and "Nihilanth". In the Opposing Force expansion, a Gargantua can be seen in the chapter "Foxtrot Uniform" tied up on top of the hydroelectric dam, while marines and Race X Shock Troopers fight around and against it. A Gargantua also appears during the final sequence of the Half-Life demo, Uplink, where it kills several Black Mesa personnel before slowly heading towards the cornered player, at which point the demo ends.
In Half-Life 2: Raising the Bar, an early script for the beginning of Half-Life 2 mentions that "something that resembles the old Gargantua" was going to be seen from the train ride in the opening scene of the game. However, this introduction scene was heavily cut down throughout development, and the Gargantua is not in the final game.
An Ichthyosaur is a carnivorous creature that can only swim underwater. It has a fish-like tail and monstrous features: It has a head larger than a human's, enormous fangs and claws at the end of its arms rather than fins. It can be defeated quickly by a few shots from a tranquilizer crossbow.
The Ichthyosaur was often featured in the original Half-Life and its expansion packs and was briefly seen once charging towards Gordon Freeman when Dr. Kleiner's plan of escape by teleportation goes wrong in Half-Life 2.
The Grunt is a semi-sentient, burly alien species that comes from Xen. It sports the deadly HiveHand weapon, which fires deadly, small homing insects, and also has purple, metallic armor, consisting of shoulder pads, a cod piece, back armor, boots, and a helmet. It also averages around 7 feet tall, making it rather intimidating. They can work in squads or pairs as well making them one of the more dangerous enemies around. By the end of the first game, the player can see Vortigaunts working near pods that if broken, will cause Grunts to appear, suggesting they are artificially made or matured.
Snarks are small, insect-like beings of a deep red hue, approximately the size of a human hand. They have a single, green eye on the front of their body. Snarks have four legs, and a pair of pale blue-green pincers on the front of their bodies that they use to attack. In the game, the player is able to use snarks as weapons, by hurling them at enemies, and in the multiplayer version of the game, players can hurl snarks at each other, as in the single-player game. The creatures soon tear through seemingly most organic materials, and so they kill their human victims within a matter of seconds. They are fast and can leap very high, so as to bring down larger opponents. Snarks seem to be physically and chemically unstable, as they explode into a green goo after a set amount of time after being launched. Shooting or hitting them will also make them "pop".
On the multiplayer game map named Snark Pit (snark_pit), the eponymous snarks are contained within a metal pipe at the top of the area, which is made up of four levels, each of which can be accessed by stairs or ladders, as in other maps on Half-Life. The pipe overhangs successive holes which are in line with each other, and therefore anything dropped or thrown down will fall through the holes consecutively and reach the bottom area within a short amount of time (however, this is not recommended for players, since the impact of the fall will kill a player) The player can activate a button near to the pipe to open it and thus release snarks into the lower areas of the map, meaning that on multiplayer, anyone spawning or respawning at the bottom will almost certainly be chased by snarks. After a set amount of time, the snarks will explode and the pipe will close. For some unknown reason, the player only seems to be able to release the snarks once in a game.
A Gonarch is one of the most powerful creatures inhabiting Xen; it is the final stage of the headcrab. Only a single Gonarch is encountered by Gordon Freeman in Half-Life. In the original Half-Life, it is located within a chapter named "Gonarch's Lair" set in the border world Xen, and acts as a mini boss in a protracted battle that takes place in the closing stages of the game. The Gonarch is similar to a spider, resembling the B'omarr monk Star Wars creature seen in Return of the Jedi. It has four legs attached to a thick disc that has eyes all around the edge. The disc also carries a large sac beneath it, which holds headcrabs that have not yet reached full size. The headcrabs are smaller, faster and coloured pale yellow. The player must defeat Gonarch by continuously shooting the sac underneath it. The Gonarch will attack with a sort of blue/white chemical blasted in a trajectory line from atop its head. In close combat, the Gonarch uses its pointed legs. After the Gonarch sustains enough damage, it will flee to another area, where the same thing will happen. After several areas, the Gonarch will finally fall into an underground pit, where the player has the option to shoot at one of the strange non-sentient tentacle-creatures near the Gonarch (part of the xen fauna, which cannot be killed) to kill the Gonarch without using much ammo. Once killed, in its death writhing, the Gonarch falls down into another smaller pit where a portal orb is created and Gordon must come in contact with it to continue to the next level.
The Nihilanth is the primary antagonist and the final boss in Half-Life. In the game, the Nihilanth is the controlling intelligence behind the Xen invasion of Earth. It communicates telepathically with Freeman through cryptic messages while he is in Xen. Hovering silently in an enormous cavern, it has been commanding the forces of Xen and holding open a dimensional rift connecting Xen to Earth, enabling its creatures to attack.
Physically, the Nihilanth resembles a gigantic, oddly-proportioned fetus with its head bigger than the rest of the body, and its brain cavity forming most of the head's bulk. Its legs, minuscule in relation to its body, appear either to be vestigial or the remains of amputation. The creature sits on a large mechanical levitating contraption consisting of a large base with spikes underneath. Like the rest of the sentient Xen races, the Nihilanth has a third appendage protruding from the middle of its chest, suggesting evolutionary lineage with the Vortigaunts. Its unusually long arms end in disproportionately huge hands and fingers, and it wears metallic wristbands, which look similar to bands worn by the Vortigaunts.
The Nihilanth attacks by launching energy balls, which come in two types: a swarm of purple-colored electrical spheres that causes immense damage, and a slow-moving, green-colored orb (similar in appearance to the teleportation portals constructed in the Black Mesa Research Facility) which homes in on and teleports its target to other sections of its lair populated by Xen aliens. It also uses these green orbs to teleport Vortigaunts and other Xen parasites into its main chamber to help defend it against intruders. A series of gold-colored "energy spheres" orbit around the Nihilanth's head, rendering it impervious to damage. These are consumed as the Nihilanth suffers damage from the player, but crystals resembling the one at the start of the game within the upper portion of its chamber replenish the spheres as they vanish.
A Chumtoad is a toad-like Xen parasite; it was originally designed for the first game of the series, Half-Life, but not included in any official game until one cameo appearance in Half-Life: Opposing Force and two in Half-Life: Blue Shift. It has not appeared in any official game since. The Chumtoad is also known by its less-fortunate name "Choad".
Chumtoads are small, amphibious parasites found on Xen. Similar in morphology to Earth's toads, they are very low on the Xen food chain – serving as the favorite prey of many Xen parasites. Chumtoads have a bright purple coloration, and a single red, catlike eye, spiked backs, and long, blue tongues. Unlike most Xen parasites, Chumtoads are not inherently dangerous to humans. They tend to lounge on rocks until threatened, at which point they can teleport, vanishing in a puff of purple energy. Chumtoads maneuver with short, quick hops, and can swim and leap to great heights if startled.
The original role of the Chumtoad in Half-Life was as bait that a player could use to lure or distract enemy organisms. The bait idea was never developed, and the Chumtoad was eventually cut from the game. The unused model, however, was included in the game's files and fully coded as an item. Instead of bait, this version served as a weapon. Normally docile until picked up, the Chumtoad would begin attacking anything that strayed too close – including the player or allied NPCs if no other targets were available. This AI was implemented in the final game as the behavior of Snarks. The source files for the Chumtoad are included in the Half-Life SDK, where it is referred to as a "Chubtoad". According to the textures provided, the Chumtoad was once planned to have six eyes instead of one, and would have had green, orange, and blue-colored variations.
The Archer is an alien cut from Half-Life. However it can be found in the game files. The Archer's model has the size of a small shark. A fish-like creature, it was to be met underwater. According to its animation, it was to bite the player, and shoot from its mouth an unknown substance, possibly a toxic spit or water-based projectile, an attack also seen on the Bullsquid.
The Charger is an alien cut from Half-Life. While not in the game files, its source files can be found in the Half-Life SDK files, but the animations are missing. The Charger was to be a two-legged insect-like creature, with a small head dotted with two moving flaps[possibly ears, since holes can be seen in the inner sides]that could cover its head completlely. It was to have seven eyes, three big and four small, randomly spread around its face[or only three, depending on the two texture files], six pointed teeth, and overall colors similar to that of the Bullsquid.
The Fast Walker is a dog-like alien ally cut from Half-Life. It never went further than the concept stage and was therefore never scripted, due to a lack of convincing AI and clear gameplay goals. Highly intelligent, the Fast Walker was to be an ally of Gordon, learning to love him along the way for all the fresh meat he would inadvertently provide through combat. With the help of its six eyes (like the original Chumtoad design), which give it night vision and heat detection, it would go before Gordon and bark to give him clues about what was coming around the next corner. He was also to have well developed limbs allowing for the use of machinery and weapons.
The Flocking Floater is an alien enemy cut from Half-Life. It can be found in the game files. Described as a "living balloon" with a faintly glowing head, it is held aloft by buoyant gases trapped in its head membrane. The wing on its rear portion is fully articulated like a bat wing, tipped with poisonous spines, and serves as a sail. It was to be seen at least on Xen. It has feathery antennae, much like a moth[its only sensory organ], "tasting" air to look for group mates or food. Passive when not attacked, it was to use an orifice to shoot projectiles at the player with gas when attacked.
The Kingpin is an alien enemy cut from Half-Life. While it lacked convincing AI and gameplay goals, it was scripted nevertheless. It can be found in the game files. Twice the height of an average person, it was to have a massive brain filling its entire body, with four eyestalks spaced evenly around it. An early version was to walk on three chitinous legs with a clicking sound; a later version traded the rear leg for a thick worm-like "tail". It was to be found on Xen, in some caves. Its psionic attacks and psionic defense would have made it immune to missile weapons, although this is unconfirmed.
Mr. Friendly is an alien enemy cut from Half-Life. It can be found in the game files. Mr. Friendly's model is the size of a small horse. It was to be a crawling creature walking on hoofed legs on its back and spurs on its front, moving in a very awkward, shuffling manner and making a noise resembling "fingernails on a chalkboard" as it moved around. Its most recent attacks in the game's development are shown in the model animations. They show it standing on its rear legs and slapping the player with its front arms, as well as vomiting. The creature was also to literally "rape" the player, holding them with its tentacled arms until the point of fatal copulation, the reason for his strange name.
The Panthereye is an alien enemy cut from Half-Life. It comes in two versions: a blue-colored one, which was created first, and a red colored one. While not in the retail game files, the source files of the red version can be found in the Half-Life SDK files, where its folder bears the name "Diablo". Both the blue and red versions can be found in the leaked Half-Life Alpha. Like many of the Xen creatures, it has a single eye and is overall very similar to the Vortigaunt. Originally created for Quiver, it was first called "Sound Dog". This theme relates to the Houndeye, who bears a similar name.
The Hazardous Environment Combat Unit (HECU) is a fictional U.S. Marine Corps special operations detachment in Half-Life created to deal with dangerous environments not suitable for ordinary forces. During the Resonance Cascade, they are deployed in massive force to the Black Mesa Research Facility to eliminate the alien threat and cover up the incident by eliminating the Black Mesa staff. However, the job is too big for them. Running low on supplies and taking constant casualties from the never ending waves of Xenian Aliens, they begin an immediate retreat. All military personnel are ordered to evacuate the base. Many don't make it to the rally point in time and are left to die. Of those left behind during the evacuation, Corporal Adrian Shephard is the only known survivor.
The HECU use a variety of weapons and equipment to get the job done. They use M34 2-1/2 ton cargo trucks, M2 Bradley IFVs, and V-22 Osprey Helicopters to deploy troops. M1A1 Abrams MBTs, M6 Linebackers, and AH-64 Apache Helicopters are used as light support units. The Infantry primarily use submachine guns (which are turned into assault rifles in the HD pack) with an underslung grenade launcher attached, shotguns; and in Half-Life: Opposing Force, light machines guns. Infantry units are issued high-tech Powered Combat Vests to protect them from a variety of hazards including gunfire, explosions, electrocution, and toxic chemicals. On one occasion, Gordon is able to call air strikes from an abandoned HECU post, although air strikes are never used against Gordon in the game.
Leeches are worm-like parasites that survive only underwater. They are very small but aggressive and try to take a bite out of a human. Once a leech is out of the water it can barely move. By one hit from a crowbar it will immediately die.
It is never revealed that Leeches are creatures from Xen. They are often found in floods in Black Mesa in the original "Half-Life". In Half-Life 2 and Half-Life 2: Lost Coast leeches make an appearance as a stage hazard to keep players from traveling too far from the mainland in the coastal areas. The Leeches will appear in large swarms and continually attack the player until they return to the mainland or are killed. Leeches are a sign of the poor condition Earth is in after the Combine took over. Most of the ocean is full of large swarms of leeches and few, if any, fish or other marine wildlife.
The Black Operations (or Black Ops for short) is a special operations unit sent by the CIA mentioned in the first game and its expansion packs. They are elite female operatives that are rumored to work independently from the HECU. The soldiers are mostly uniformed in black sometimes with red night vision goggles. They are often very silent, fast, and can become invisible. They too as well as the HECU have come to infiltrate the Black Mesa facility, solve the situation, and assassinate the hostile creatures and the human witnesses.
It isn't until the closing stages of Half-Life: Opposing Force that they reveal 'the package', a nuclear explosive device that will contaminate the entire Black Mesa base. HECU corporal Adrian Shephard locates the bomb and disarms it not long before the G-man appears and rearms it after Shephard is gone.
Although they are rarely featured in Half-Life, they play a bigger role in Half-Life: Opposing Force. Gordon Freeman first encounters them before the HECU captures him, and encounters them one last time while en route to the Lambda complex where he faces them just as he finally arrives there.
Black Operations can again be seen in the Decay bonus mission Xen Attacks where two Vortigaunts try to retrieve Xen crystals deep in the basement of Black Mesa.
The Black Ops soldiers' equipment are nearly equivalent to the HECU's, e.g. assault rifles and turrets. They also have the same shaped vehicles such as military vans and attack helicopters but black in color. They are the only ones that use Glock pistols (Glock 17 type).
Race X is a hostile, non-Xen alien race. They appear only in the Opposing Force expansion pack, and very little is revealed about them. Race X aliens appear late in the timeline of Half-Life, a few hours before Gordon Freeman teleports to Xen.
Pit Drones are the second type of Race X aliens encountered in the game. They are short bipedal creatures with yellow-tan skin and a small head with rows of sharp spines. A Pit Drone's arms end in long, scythe-like claws, which it can use to make vicious attacks. Fast and fairly intelligent, they tend to appear in groups of five or six, and are capable of making flanking movements and quickly surrounding their prey. They have two forms of attack: firing their head-spines like darts through acid jets when at long range, or charging towards the target to deliver a deadly slash with their claws. Some Pit Drones appear not to be capable of the head spine attack and, on closer inspection, have no spines in their head. It is notable that when Pit Drones are first encountered, they create light thumping footsteps while dashing through a long vent grate above a garage area.
Shock Troopers are the first encountered and most intelligent of the Race X aliens. Physically, they are humanoid with four arms and have a posture that makes them vaguely resemble Vortigaunts. Their bodies are covered in plate-like sections, similar in appearance to insect exoskeletons. Their skin is pale blue, and is not mottled or patchy like the skin of Xen creatures. They have a single large eye, like Vortigaunts. This eye is apparently part of a common morphology between Race X creatures - every Race X alien with a large eye (including the Pit Worm) has two eyelids which regularly blink horizontally across the eyeball and what appears to be "tentacles" surrounding their mouth (which is under the head).
Their primary weapon is the Shock Roach; a living creature, similar to the Alien Grunts' hive-hand. The Shock Roach fires bolts of electricity, which it constantly recharges on its own. The Shock Roach cannot survive without a living host for very long; if its Shock Trooper dies, it will detach itself and pursue a new host. If it nears a player that already has a Shock Roach, it will attack before dying.
Shock Troopers also have the ability to regurgitate and throw spore grenades, which they tend to use against distant or concealed enemies; they also have a very lethal melee attack that is capable of tearing Shephard to pieces by scratching him with their arms. This 3rd attack is only used when approached toe to toe. Infant Shock Troopers will forcibly launch the same type of spore grenade when squeezed, and can thus be used as a Spore Launcher weapon. In one of the Spore Launcher's idle animations in Opposing Force, it emits an amphibious purr, at which point Adrian Shephard strokes the top of its back and forth as if it was his "pet".
Baby Shock Troopers are effectively tadpoles, an earlier stage that is only a head and torso, with tentacles trailing behind. Beyond the small size and lack of limbs, Baby Shock Troopers also have gills and two small eyes instead of the adults' single large one. They are amphibious, although immobile outside water. Baby Shock Troopers are first found in a tank in the Sector E Biodome Complex of Black Mesa. Shock Troopers are usually encountered in small groups, and fight together with other Race X aliens. Shock Troopers have a verbal form of communication. Shock Troopers use teleportation technology to travel in and out of combat. They also control the teleportation of Pit Drones and Voltigores.
Voltigores are gigantic, lumbering creatures serving as the race's heavy support infantry. Voltigores have four legs and two arms that end in scythe-like claws, and are distinguished by purple stripes on their bloated carapace similar to those on a houndeye, though in a different color. These aggressive creatures use their innate ability to harness electrical charges to create a devastating ranged energy ball attack, bearing vague similarity to the Vortigaunt's green energy stream, although the Voltigore's energy attack is purple, an avoidable projectile and is more powerful. At close range, they attack viciously with their claws. A player can recognize a Voltigore from afar by the unusual, electric-sounding bellows of the creature. When a Voltigore dies, its carapace explodes, causing massive damage to nearby creatures. Baby Voltigores appear later in the game. They seem to be adapted from egg-shaped shells the Voltigore lay in caves or tunnels. They are smaller, weaker, lighter in color, and use the energy attack in a lighter way.
Encountered in a sewage processing area in the lower levels of Black Mesa, the Pit Worm is composed of a segmented body which supports a head dominated by a single large eye. This eye is the pit worm's primary weapon, capable of firing a searing energy beam at nearby enemies. If enemies get too close, the worm also has large claws to strike with. If the worm is shot in the eye, it recoils, shielding it with its claws for a few seconds before striking aggressively at the player.
The Gene Worm is the final boss of Half-Life: Opposing Force. This life form is the equivalent of a "biological resource management factory". It is capable of taking any of Earth's natural resources and assimilating them to make them useful to the aliens' needs. If one fully entered Earth and became productive, it would effectively alter the planet enough to become useful as a new homeworld for Race X.
Physically, the creature resembles a green, worm-like creature with many tentacles, with a beak-like protrusion similar in both shape and position to that of the Xen tentacle. The Gene Worm attacks with its tentacles and also ejects a deadly chemical stream from its mouth. Its outer hide is impervious to damage; in order to kill it, the player must blind the creature by shooting both its eyes with the mounted laser guns on either side of the worm. This causes it to summon reinforcements by opening its abdominal cavity, revealing a portal orb that a Shock Trooper will teleport through.
The Gene Worm is found deep underground in a seemingly abandoned industrial area of Black Mesa, and appears through a portal opening in a room there. The room itself has been fitted with mounted weapons which are powered by Xen energy crystals. The player character, Adrian Shephard, is sent down to the portal by a Black Mesa security guard, who remarks "Some of your buddies went down earlier, and I haven't seen 'em since". Ten years later Gearbox would re-visit the Gene Worm encounter as another final boss in their 2009 video game Borderlands.
Taking on the appearance of what looks like a flying ball of light, Sprites are creatures that can be seen floating through the facility several times during the game, becoming more frequent in later stages of the incident. However, they cannot be harmed and do not hurt the player in any way. On occasions they even seem to be guiding Adrian. In the scenario where the Gargantua is trapped on the Sector G hydroelectric dam, sprites fly into a portal nearby which the G-Man then steps through. They eventually all lead to a massive violet portal that brings the Gene Worm in. At the time Gordon Freeman pushes the Xen crystal into the Anti-Mass Spectrometer, many Sprites fly towards and away from the resonance cascade, the former of which also happens when the portal to Xen in the Lambda reactor core is fully opened.
Antlions are an insect-like species that first appear in Half-Life 2. Antlions are similar to ants in their "hivemind" behavior. All types of Antlions are found in the Nest in Half-Life 2: Episode Two. Here are the types of Antlions that appear in the games:
Antlions are green striped creatures that were discovered in beaches along the coast near City 17. Regular Antlions are not much of a threat alone, but endanger the player when in groups. Antlions appear to possess some intelligence in that they are the only non-human enemy that will actively try to flank or surround the player. In the chapter "Sandtraps", it is discovered that, if the player steps on the sand, it "makes the Antlions crazy" and makes them come out the surface and attack. You also learn that vibrations, usually made with Combine Repulsion Stations (also known as "thumpers") will repel Antlions. On Highway 17, parking near a CRS provides a reprieve when the number of antlions attacking the player gets too high. After the player kills the first Antlion Guard he or she fights, a Vortigaunt comes and removes the Antlion's pheropod, informally known as "bugbait". The player can use the bugbait to call on the Antlions as allies, as they are attracted to the pheromones, and no longer see the player as a threat. Antlions can provide great help for Freeman while he's infiltrating Nova Prospekt, as they are quite effective at slaughtering Combine troops and getting at them in hard-to-reach places like towers due to their grasshopper-like jumping ability. In Half-Life 2: Episode One, Antlions come into City 17 because the Combine Defense field was destroyed. Antlions come through burrows like in the level Nova Prospekt. They are also hostile creatures to the Headcrabs and the zombies the Headcrabs produce.
Antlion Guards, also known as Myrmidonts, are among one of the most feared creatures and the most powerful in the Half-Life universe, second only to Gonarch. Their aggression surpasses that of the regular Antlion. They are larger and more armored, and faster than the regular Antlion itself. Antlion Guards prefer to charge at the player, and will attempt to headbutt the player. They are also able to throw objects such as cars at the player, and are the only organic NPC in the game that can't be destroyed with a single hit from the AR2's alt fire. A portion of the second chapter of Episode Two is spent running from a version of the Guard. See "Antlion Guardian" for details.
Antlion Grubs are small, glowing worm-like larva that usually reside in Antlion nests in Half-Life 2: Episode Two. They affix themselves to walls, ceilings or floors and do not move. Grubs are very weak, and they cannot attack. They are easily killed with one bullet, or even by being stepped on. When killed, they drop small yellow "grub nuggets" that restore one to six points of health to the player, depending on the size. Killing every grub in Half-Life 2: Episode 2 will earn the player an achievement.
Antlion Workers or Acidlions are luminescent types of Antlions, though they are stronger, spit acid, and resemble termites. They also are somewhat more defensive in their attack strategy: they will stay at range and retreat when advanced upon, and are capable of flanking the player or leading the player into traps. Antlion Workers are usually found in the Antlion Nest in Half-Life 2: Episode Two. Like all Antlions, Workers also hate thumpers. Usually the Antlion Workers are the ones guarding the Grubs.
Antlion Guardians are similar to the Antlion Guard, but are green and have a neural-poison attack when underground. The Guardian guards the path to the Extract, which is used by the Vortigaunts to heal Alyx Vance after she is wounded by a Hunter in Half Life 2: Episode Two.
The Combine, also referred to as the Universal Union, is a fictional multidimensional empire, which serves as the primary antagonistic force in the Half-Life video game series, developed by Valve Corporation. The Combine consist of alien, synthetic and human elements, and dominate Earth. They are regularly encountered throughout Half-Life 2 and its episodic expansions as hostile non-player characters as the player progresses through the games in an effort to overthrow the Combine occupation of Earth.
The Combine are frequently shown as harsh rulers over the citizens of Earth, suppressing dissent with brutality, policing using violence and using invasive surgery to transform humans into either soldiers or slaves. Throughout the games, the player primarily battles with transformed humans as well as synthetic and mechanical enemies that are the product of Combine technology. The atmosphere generated by the dystopian Combine state has been praised by reviewers, although the artificial intelligence of the transhuman Combine characters was thought to be inferior to that of other characters in Half-Life 2. In addition to their role within the Half-Life series, the Combine have been adapted for machinima productions and one synthetic enemy character of the Combine has been made into a plush toy by Valve.