The player can choose from one of four characters who have different back-stories and who seek to reestablish the 10th Legion and fight Jeyne Kassynder for control of Ehb. The background is based in a fantasy medieval world with geographic variety in open and closed environments (dungeons). Following an ambush by Kassynder's forces in the beginning, the player character will travel through Ehb in order to fulfill their goals.
As the game progresses and different quests are completed, the other three characters will first meet and then join the player character. Depending on which character the player chose, some events encounters might be subtly changed (for example, while Lucas, Anjali and Reinhart will meet Katarina for the first time in the town of Raven's Rill helping them, players who chose to play as Katarina will meet a non-player character serving the same purpose) and the lines of dialogue are altered to fit the character's back-story. Unlike the previous games in the series, the player can only choose a single one of the three non-player characters to accompany them at a time, although they can exchange them immediately via the game's menu at any point of the game. The companion character will be controlled by the game's AI but can be seamlessly controlled by a second player through the game's online feature. Using the multiplayer feature, up to four players can play the campaign together.
Each character has two different styles of fighting, each to handle different situations. For example, the character of Lucas can wield a two-handed sword to damage a lot of enemies at once but with less protection or use a one-handed sword and shield to fight against single strong enemies or to block attacks. In addition, there are three different special abilities for each stance that require "focus", the game's equivalent to magic points, to use. The amount of focus required varies based on the ability used. Each of those abilities has an improved version available after meeting certain conditions, such as using the skill a certain number of times. There are also three different passive abilities (such as healing, increased armor etc.). The improved versions of the attack abilities and the passive abilities use a different system of power spheres. All improved and passive abilities will deplete such a sphere. Both the focus points and the spheres are refilled by fighting enemies. While amount of focus available will always be a maximum of 100%, the player will start with only one sphere and gain three additional ones at key points of the plot, regardless of their level or experience points. Additionally, each character has ten different skills they can increase at each level reached, allowing them to improve their attacks, get bonus effects when being hit, etc.
Each of the four characters features nine different sets of equipment, meaning that (with the exception of rings) no piece of equipment can be used by multiple characters. It also features an "transmute item" mode, allowing the player to transmute their items into their worth in gold instantly from the items menu without having to sell them to a shop (although transmutation will yield a lower price).
Unlike the previous games in the series, Dungeon Siege III features a new control system. Created to accommodate the parallel release on consoles as well as on the PC, the game no longer features an automatic targeting system and point-and-click mechanics. Instead, the player moves their character using the keyboard or a controller and has to point the character in the direction of the enemies they want to fight and then press the attack button to fight.
The game uses a display engine that makes the entire experience seamless by constantly loading required game scenes in the background. This way, the game does not need any load screens often found in other games.
The game returns to the Kingdom of Ehb, which already featured as the setting of the first (but not second) game. Roughly 150 years after the events of Dungeon Siege, the 10th Legion, who provided stability and protection for the kingdom for 400 years, has almost completely been wiped out. 30 years before the game's setting, Jeyne Kassynder rallied the people of eastern Ehb and the Azunite Church against the Legion who she blamed for the death of the king (her father). Following a long and brutal campaign, she managed to kill the entire remnant of the Legion in a battle in the Rukkenvahl Forest and the royalist army of young Queen Roslyn has been driven deep into the mines of Glitterdelve, losing almost all of their power.
A small group of legionnaires remained, led by a man calling himself the Venerable Odo, a former Legion spy. He kept descendants of the Legion safe from Kassynder and, at the beginning of Dungeon Siege III, called them to meet in order to regroup the Legion. The player character arrives to find the meeting place attacked by mercenaries of Kassynder's. With the help of Marten Guiscard, another Legion descendant, the player tracks down Odo at a former Legion safehouse. After hearing what happened, the player sets out to find other survivors and information on the attacks. After fighting more of Jeyne's mercenaries, the player travels to Stonebridge City, to reopen the grand chapter-house there. On the way to Stonebridge, they help a group of royalist soldiers fighting Jeyne Kassynder.
After reopening the Stonebridge chapter-house, the player seeks to enlist the support of the royalists by aiding Queen Roslyn against Jeyne's assault on Glitterdelve and the help of the City of Stonebridge by helping the Meisters who control the city. Their support ensured, the player attacks Jeyne's capital, the Spire of Azunai on top of Mt. Jhereb. After winning in battle against Jeyne, revealed to be an archon, a member of a servant race to the lost creator gods, it is revealed that she seeks to use her power to reawaken her long lost gods in the Rukkenvahl Forest. The player travels to Rukkenvahl and fights a mutant god created by Jeyne. After the battle, the player captures Jeyne alive and may render judgment on her, allowing the player to choose the ending.
Dungeon Siege III features four different characters. Depending on which character the player chooses, the plot is subtly altered, with the other three characters later available as AI controlled companions or as characters for other players via the Multiplayer feature.Lucas Montbarron: The youngest son of Hugh Montbarron, the Grand Master of the 10th Legion at the time of their last fight against Jeyne Kassynder 30 years before the game's setting. He is also a descendent of the first Lady Montbarron, the protagonist of Dungeon Siege, who is commonly only referred to as "The Farmer" (Dungeon Siege III takes place 150 years after the first game). Lucas fights with a one-handed sword and shield or a two-handed sword.
Anjali: An archon, a member of a servant race to the lost creator gods. As a mythical being, she can shift between human form, fighting with a spear, and elemental fire form.
Reinhart Manx: A descendant of Merik the mage (from the first game) and therefore is able to fight with magic at range and in close combat.
Katarina: The illegitimate daughter of Hugh Montbarron and a Lescanzi witch. She fights with a long-range rifle or a pair of shotguns at close range. Rebecca Grant modeled for Katarina's character.
Chris Taylor, the original creator of Dungeon Siege, served as an advisor during the development of the game. It is the third game in the series not to be developed by Gas Powered Games (after Dungeon Siege: Legends of Aranna by Mad Doc Software and Dungeon Siege: Throne of Agony by SuperVillain Studios) and the first to be published by Square Enix after it acquired the complete ownership of the Dungeon Siege franchise. After the release, a number of players complained about the controls on the PC version of the game, prompting the developer Obsidian Entertainment to address those concerns.
A three-issue mini-series also titled Dungeon Siege III was released digitally by Dark Horse Comics. A printed fourth edition was available as a pre-order bonus from Walmart.
Dungeon Siege III: Treasures of the Sun was released on October 25, 2011, as "downloadable content" (DLC) for Dungeon Siege III. It features a new location, several new monster types, three new obtainable perks, and raises the level cap to 35. The story follows the protagonist(s) on a journey to Aranoi desert, where an undead outbreak is taking place, in search of a long lost Legion spy and the mysterious "greatest treasure of the Azunite church". According to Destructoid, the downloadable content offers approximately five hours of added play time to the base game.
The title received a lukewarm response from critics, earning a Metacritic score of 72, 71 and 73 for the Xbox 360, PlayStation 3 and Windows platforms respectively. Charles Onyett of IGN stated that "Obsidian Entertainment's take on the Dungeon Siege franchise has some merit. The core combat system is well-designed, offering a small but useful set of abilities for each character class. The varied enemy encounters encourage the frequent use of abilities and the dodge mechanic, which makes the basic gameplay whittling down the health bars of enemies satisfying. But the rest of the game mostly falters, unfortunately, to a degree that overshadows its achievements. Dungeon Siege III is a dungeon crawler with a boring loot table, poorly implemented multiplayer, and little lasting incentive to continue running around the world once the tedious story ends." Game Informer states that the graphics are dazzling and the character models are beautiful especially when they are zoomed in on. GamesMaster criticised the repetitive nature of the gameplay but acknowledged that the multiplayer co-op was enjoyable.