The Tau are oriented towards ranged combat and generally die quickly in close quarters. They have some of the most powerful ranged weaponry in the game in terms of both range and stopping power, with their railgun being the most powerful of all.
The Tau do not have any psykers or units that specialize in countering psykers, which makes them somewhat more vulnerable to psychic attacks.
All Tau vehicles are classified as flyers, skimmers, or jet pack infantry, meaning they can all move swiftly over difficult terrain.
The Tau are the only army in the game that routinely incorporates aliens. Kroot warriors provide melee support, while the insectoid Vespids serve as jump infantry.
The Tau were released in October 2001. Unlike most of the races in Warhammer 40,000, which were developed from a comparable race in Warhammer Fantasy Battle, the Tau, along with the Tyranids, are the only playable races that do not possess an analogue in the Fantasy fictional universe, although most of the combat doctrines are based on the Wood Elves or Dwarfs.
The Warhammer 40,000 Design Team selected the Tau as one of three new race ideas from hundreds of possible concepts. The Kroot were one of the others, and these two were eventually combined into the one fictional organisation; the Kroot were later given their own army list written by Andy Hoare in the Chapter Approved 2002 publication. This list permitted the Kroot to be used as mercenary forces for a selection of other races or as a stand-alone army.
According to Andy Chambers, the chief designer at the time, the Tau were intended "to be altruistic and idealistic, believing heartily in unification as the way forward." Graham McNeill was responsible for much of the background material produced for the Tau, developing what Andy Chambers described as "their proud, quiet but determined character [developed] to the point where they actually became a rather likeable, if slightly naive addition to the cosmos."
This development was eventually seen as too disparate from the traditional dystopic atmosphere of the rest of the setting. The Tau were eventually modified to display the same altruistic overtones, but with a heavier Orwellian tone that implies that the Tau engage in mind-control and population replacement on worlds within their domain.
Tau miniatures were designed to display the high-tech science fiction and robotic concepts that had resulted in the choosing of the Tau as the new army-race. The reflection of the Tau's high-technology status was reflected by the lack of cabling and links modeled onto the weapons; instead it was decided that these components were internally integrated. The Tau Infantry models, according to sculptor Jes Goodwin, were designed to have subtle influences taken from Chinese foot soldiers. The Battlesuits and vehicles drew from science fiction exo-suits, and were designed to slightly resemble a faster and more lightweight version of the Space Marine Dreadnought. While the Tau vehicles are 'skimmers', the design brief specified that the Tau Tanks have an impression of being heavier and more solid than the Eldar Grav-tanks while nowhere near as solid as some of the more heavily armed vehicles like those owned by the Orks or the Chaos Space Marines.
The primary weapon for Fire Warrior teams are the Pulse Rifles, and the Pulse Carbine, a smaller, shorter-ranged version of the Pulse Rifle that is equipped with an underslung "photon grenade" launcher that can pin down enemy infantry. Both of these weapons function by firing particles that break down into plasma pulses as they are fired. Notably, the Tau are at an extreme disadvantage when it comes to close combat, as their standard infantry lacks any melee weapons.
The Rail Gun is the most iconic, and feared, of Tau weapons, which can be found on the Hammerhead tank (as of the latest codex, the "heavy support" Broadsides do not sport railguns, but heavy rail rifles). The rail gun has extreme range and is immensely powerful, being one of the best weapons in the standard Warhammer 40,000 game; its effect upon an Imperial tank is described as two neat holes in both sides, the crew sucked out through one of them as the projectile exited, their remains scattered across two dozen meters.
Since the setting of the Eye of Terror Worldwide Campaign was on the opposite side of the galaxy from the Tau Empire, and published materials had previously established that the Tau have limited faster-than-light capability, a separate 'mini-campaign' was held specifically for Tau players. Codex: Tau Empire (Hoare, 2006) was the first publication to incorporate the impact of this game event on the 40k universe. In the new background material published with the Codex, it is explained that Imperial forces were drawn away from Tau space to defend against Abaddon's Thirteenth Black Crusade. This left a power vacuum that prompted the Tau to initiate their Third Sphere Expansion.
In the campaign, registered games involving the Tau contributed to the expansion or contraction of Tau-controlled space. Over eight weeks of gaming, the Tau Empire grew by nearly a third due to victories.
The Tau were the fourth army to receive a Codex updated for Fourth Edition rules (Codex: Tau Empire - Hoare, 2006). Additional rules for the Tau appear in a Forge World Imperial Armour rules supplement (Imperial Armour Volume Three - The Taros Campaign - Kinrade, 2005).
The Tau evolved as hunter-gatherers in the arid plains and desert environments of their homeworld, T'au, though they eventually spread to all ecological regions of the planet. As a result, they have tough, leathery blue-grey skin, which exudes no moisture. With an average height of 5'5", they have a humanoid body plan, though unlike humans they possess digitigrade legs which end in cloven hooves. The Tau have flat, nose-less faces, with their olfactory organs located inside of their mouths to preserve moisture. On the battlefield, the Tau typically wear all-concealing battlesuits which give them an even more alien or robotic appearance, but a Tau soldier that takes his helmet off for a closer look will reveal that their cranial arrangement isn't that much different from a human's (compared to say, a Tyranid; they don't look as similar to humans as the Eldar). The different Tau castes have slightly different body proportions between themselves.
The Tau possess no psykers, and are said to register as little more than a "blip" in the Warp. Tau ships have no navigators (a psyker used for warp travel) and only realized the existence of the warp after contact with the Imperium. Instead the Tau make use of a form of Hyperdrive, a slower type of FTL travel and in turn drastically slowing the spread of the Tau across interstellar space. Their inaccessibility to the warp has also denied them the combat abilities of psykers on the battlefield, with the somewhat dubious benefit that they have very few encounters with the forces of Chaos. Indeed, there has never been a recorded instance of a Tau being tainted by Chaos in any form. (However, in Fire Warrior, the only Black Library published Tau novel, one Shas'la, La'Kais, is indeed tainted by a vast Daemon Lord. The novel explains that, rather than lack of psychic ability, it is the lack of selfish desires linked with Tau'va, the Path of the Greater Good, that protects the Tau from Chaos. La'Kais lacked this; thus he alone among the Tau force was rendered vulnerable to Chaos). Their negligible signature in the Warp is also one of the main reasons that the Imperium of Man's psykers ignored the Tau for thousands of years, when they were seen as primitive hunter-gatherers confined to their own insignificant planet. Thus, the Imperium was taken by surprise when the Tau started building their small but vigorous interstellar empire, driven not by psyker adepts but by advances in technology, which the Imperium utterly shuns.
Tau society is divided into five castes. They are named after the elements of nature, which reflect the characteristics of each caste.
Originally, all Tau were hunter-gatherers who evolved on the arid plains and deserts of T'au. The culture of all Tau castes is influenced by terminology and mindsets related to a hunting context (to a greater or lesser degree, depending on the caste). For example, the two primary military tactics are called the "Mont'ka" (lit. "killing blow", favoring quick decisive strikes) and the alternative strategy known as the "Kauyon" (lit. "patient hunter", favoring luring enemies into an ambush).
As the Tau expanded to other ecological regions of their homeworld, the cultural lifestyle of each group differentiated based on their surroundings. Those who moved from the plains to the fertile river valleys (ancestors of the Earth caste) developed agriculture, metallurgy, and ultimately built the first true cities. With the rise of cities along major rivers came a rise in trade between cities, leading to another group of Tau becoming a culture of merchants plying their way along the major river networks (ancestors of the Water Caste). The Tau who moved to the mountain regions (ancestors of the Air caste) became very slender, and even grew membranes between their limbs which allowed them to glide on updrafts (an evolutionary trait they later lost). These became a society of fast raiders, and later, messengers between cities. Those who remained in the arid plains became fierce and skilled hunters, larger and stronger than other Tau. The ancestors of the Fire Caste, these plains-Tau became fierce nomadic raiders and hunters. Each of the groups fought the others, with plains-Tau raiding the cities of the builder-Tau, and rival builder-Tau cities attacking each other.
According to Tau legend, during the siege of the ancient builder-Tau city of Fio'taun by plains-Tau warrior-nomads, a fifth and mysterious Tau caste suddenly appeared in their midst, the Ethereals, who mediated an end to the dispute. The Ethereals preached the philosophy of the "Greater Good" and began the process of unifying the other four groups under them. The Ethereals formally established the caste system between the five groups of Tau, locking each in place within Tau society. The Ethereals forbid interbreeding, meaning that members of each caste are forbidden to change caste. A side-effect of this is that each caste has been a separate breeding population for several thousand years, each almost forming its own sub-species, leading to notable disparate physical appearances between the castes.
The Tau castes are:Fire-caste - Warriors, both ground soldiers and vehicle drivers. The caste most likely to be encountered up-close on the battlefield. Skilled hunters and fighters, they are larger and stronger than other castes (some of the other castes are taller, but do not have proportionately as much muscle mass).
Air-caste (also known as the invisible caste) - Pilots, both in the merchant fleet and staffing the military starships of the Tau Navy, commanded by Air-caste admirals. The Air-caste have been living and working in microgravity for so many generations that they have become exceedingly tall and slender, with unusually long and skinny limbs. They possess heightened depth-perception, high-G tolerance, and perception of 3D environments.
Earth-caste - Builders, farmers, miners, engineers, and scientists. The Earth-cast forms the backbone of Tau infrastructure and population. They are short and stocky, with a generally stoic outlook on life, diligently performing their work. Their labor is respected, and considered no less valuable to the overall effort of the "Greater Good" than the functions of the other castes.
Water-caste - Merchants, bureaucrats, mid-level administrators, diplomats, ambassadors and scientists. The Water-caste is the group of Tau most likely to be encountered in formal diplomatic missions sent to other races. They are somewhat taller than other Tau, though not as tall and slender as the Air-caste.
Ethereals (sometimes referred to as the "Celestial-caste") - Leaders, both spiritually and politically. Relatively few in number, though generally they seem to be about as tall as the Fire Caste, though not quite as strong physically. All Tau have an irresistible compulsion to obey the word of an Ethereal. It is not known whether this power is psychic or pheromonal. Thus, there is very little politicking among the Tau.
Ranks within Caste:
As well as the five castes of the Tau, multiple alien species are incorporated into the Tau Empire; the most significant of these being the Kroot and Vespid although many other races, including the space-faring Nicassar and the Demiurg mining fleets are members. In addition, human auxiliaries (Gue'vesa in the Tau language) are sometimes seen to be aiding the Tau as well.
The Tau Empire's practice of tolerating and incorporating other races stands in stark contrast with essentially all other major races in the galaxy, which exterminate other races completely rather than conquer and subjugate them. Reports vary on the exact conditions of the alien races working for the Tau themselves, ranging from that they are full allies within the Empire, to that they are mercenary armies hired out by the Tau to aid in the umbrella of protection the Tau military provides their region of space, to reports that these "auxiliaries" are glorified slaves. Still, the fact that the Tau want to co-exist with other races on any level, even if they are subjugated, makes them far more tolerant than other races, particularly the xenophobic Imperium of Man. Most reports generally indicate that the human auxiliaries serving the Tau Empire are relatively well-treated, with many serving the Tau voluntarily.
As the only significant human population in the galaxy which is not part of the Imperium, this has led to a great deal of both embarrassment and general bewilderment in Imperial officials, who have difficultly comprehending that many planetary-sized populations of humans would truly rather side with the Tau than the brutal and harsh Imperium. Refusing to believe that the Gue'vesa were willingly won over to the Tau Empire through what was honestly better treatment than they had under the Imperium, many Imperial officials continue to insist that other races must only serve under the Tau due to some form of mind control over their "allied" races.
Thus, in contrast to the forces of the Imperium of Man, a Tau army encountered on the battlefield may feature a wide menagerie of different alien races, working together for the Greater Good (or at least, as mercenaries working together for payment from the Tau)Kroot: close-combat troops. The Kroot possess avian ancestry and beaks, but have evolved to appear closer to feather-less reptiles. Guided by Shapers, Kroot are capable of drastically altering their genome by eating dead enemies and absorbing specific genes, to adapt to new combat niches.
Vespids: flying, wasp-like humanoids. Used as a highly maneuverable, rapid-strike force. Covered in a chitinous exoskeleton evolved to protect them from the harsh atmosphere of their home planet. Called "Mal'kor" in the Tau language (lit. "air-insects"). The Imperial Guard have dubbed them "Stingwings", though this technically only refers to their soldiers, not the entire race.
Humans - Gue'vesa (lit. "Human auxiliaries"). They fulfil a tactical niche between the close-combat Kroot and the long-range Tau Fire Warriors, in that they are capable of using some advanced Tau weaponry, but are slightly better at close-combat than the Tau. Human auxiliaries are not yet as numerous in the Tau military as the Kroot, or even the Vespid, but are being incorporated in increasing numbers.
Demiurg - avid miners and expert traders, who possess technology that is actually somewhat more advanced than that possessed by the Imperium of Man. It was actually the Demiurg who provided the Tau with Ion Cannon technology, which then became the mainstay armament used by all Tau ground forces, such as Fire Teams. The Demiurg are an entirely space-borne race which either lost or abandoned its homeworld, spreading out through the stars to mine new resources and trade. They help form a major component of the Tau Empire's interstellar economy. They are shorter than humans and have a sturdy build. The Demiurg are defensively hostile, but not aggressively militaristic, preferring to flee from stronger enemies and seek better trading opportunities elsewhere. As a result, they have generally avoided entering Imperial territory (unless they are invited in), and are thus rarely encountered by the Imperium of Man. This also means that they possess no starships designed for military purposes, and their preference for flight over fight means they do not use ground units. However, being equipped with advanced Demiurg technology such as powerful cutting lasers used for asteroid mining, even the humble mining ships of the Demiurg (such as the Bastion-class and massive Stronghold-class) are capable of successfully engaging Tyranid Hive Ships head-to-head.
Galgs - green, frog-like aliens used as mercenaries.
Hrenian - mercenaries, specializing in light infantry.
Ji'atrix - voidfarers, skilled at space-travel.
Morralian - mercenaries.
Nicassar - the first alien race incorporated into the Tau Empire. Completely space-borne, as they are too weak and immobile to be of any use on land. Insatiably curious, they explore and scout star systems for the Tau. Even in space, they have poor offensive and defensive capabilities, nor do they possess any advanced economic infrastructure, because they hibernate whenever they are not actively exploring. They are also highly psychic, though the Tau try to keep this fact from the Imperium of Man, which seeks to stringently control psykers.
Tarellians - reptilian, dog-like aliens used as mercenaries. They possess a deep hatred of humans due to the severe losses their homeworlds have taken from Imperial campaigns.
The uniting philosophy of the Tau race is called "The Greater Good", which stresses communal living and cooperation, a convivial attitude to aliens, and self-sacrifice for the good of the whole. Most Tau sincerely believe they are on a noble mission to bring peace, justice, and progress to the rest of the galaxy.
While on the surface the Tau may seem like wonderful altruists, especially when compared to the extremely brutal Imperium of Man, the fiction shows many sinister undertones. The Tau can be ruthless with alien cultures who don't fit into their utopian society. Cultures which resist assimilation into the Tau Empire are subjugated by force. The Vespids, due to their insectoid mentality, could not relate to the Tau in any way until the Tau implanted "communication helms" into their brains, which then transformed them into compliant and model citizens. Whilst many assimilated humans do enjoy more liberties and comforts than they did under Imperium rule, those who stubbornly resist are sometimes interred in re-education camps or subjected to sterilization programs.
The ruling Ethereal caste uses some form of mind control, possibly pheromone-based, to control the other Tau castes. Literally, their every command is obeyed without question, their every decision seen as wise. The origins and ultimate motives of these mysterious beings are unknown.
When the Tau first started expanding to other star systems, they thought they were the only technologically advanced race in existence, and that it would be effortless to expand their reach throughout the rest of the galaxy. In 742.M41 (the 742nd year of the 41st millennium), they came into contact with the Imperium of Man, which launched the so-called "Damocles Crusade" to conquer the upstart Tau Empire. The war lasted for three years, between 742.M41 and 745.M41, before ending in stalemate - each side agreed to a truce due to the sudden appearance of the first Tyranid invasion of the galaxy, Hive Fleet Behemoth, which threatened them both. The Imperium's forces were rapidly withdrawn from the aborted Damocles Crusade to fight in the First Tyrannic War, and in such a fast panic that many Imperial Guard regiments were abandoned inside of the Tau Empire's territory, many of whom were absorbed into the ranks of the Tau military as auxiliaries. Each side learned new lessons from the Damocles Crusade: the Imperium of Man was forced to realize that the Tau were a new major race on the galactic scene, capable of standing up to the limited resources the Imperium was able to throw at them, due to most of its armies being spread thin across the galaxy fighting Chaos, Orks, and Tyranids. Meanwhile, the Tau had a rude awakening that the galaxy was not empty and theirs for the taking, but mostly controlled by the Imperium of Man, which even with its attentions divided between multiple alien threats controlled vast military resources. In the 250 years between the Damocles Crusade and the 13th Black Crusade (by the forces of Chaos), the Tau have been slowly but steadily expanding their sphere of influence, retrenching in the face of powerful alien races.
As a civilized technological race which does not inherently embrace warfare, the Tau are one of the few races that the Imperium of Man might be considered to have "diplomatic relations" with on any level. Unlike every other alien race, the Tau do not wish the destruction of humanity, and their reach is very short due to their limited interstellar capabilities. Though the Imperium ultimately wishes to destroy all alien races, it considers the Tau a low priority and has occasionally agreed to suspend hostilities to deal with more pressing threats. Imperial and Tau troops have often fought side by side against the Necrons and Tyranids as with the Eldar, and the Imperium could essentially be said to have been allies with the Tau. Thus after many thousands of years, the Imperium of Man is slowly beginning to rediscover the concept of "international diplomacy", in which the Imperium, the Eldar, and the Tau would each prefer to conquer all of the others, but each also realizes that they cannot spare the resources for the full undertaking. Most commonly, it is simply a situation where when faced with a far more belligerent common foe that cannot be reasoned with, alliances are often in the best interests of both parties.
Due to the Tau Empire's location in the Milky Way Galaxy's Eastern Fringe, combined with their lack of psykers, they have had relatively little contact with the forces of Chaos, which are concentrated around the Eye of Terror in the galaxy's north-west, or the Maelstrom at the center of the galaxy. The Imperium of Man has for the most part not prioritized military conflict with the Tau, which does occur but at a relatively low level. Contact with the Eldar is sporadic, and relations vary from one craftworld to the next. The biggest threats to the Tau Empire come from the Ork hordes, which control vast swathes of the Eastern Fringe, as well as the incursion of the Tyranid Hive Fleets, which tend to enter the galaxy through the Eastern Fringe (ultimately driving towards Terra). Thus the Tau's two biggest military threats are alien races which possess military doctrines diametrically opposed to those of the Tau. Both the Orks and Tyranids favor devastating close-quarters combat, while eschewing ranged combat (the Orks to a somewhat lesser degree than the Tyranids).Warhammer 40,000: Fire Warrior - a first-person shooter played from the perspective of a Tau Fire Warrior, there is also a book based on the game.
Warhammer 40,000: Dawn of War
Tau are a playable race in the second expansion pack, Dark Crusade, released October 2006, as well as the third expansion pack, Soulstorm, released March 2008.