Dungeon Siege: Throne of Agony picks up where Dungeon Siege II for PC lefts off: after the second cataclysm altered the world of Aranna. A new threat appears out of nowhere, and the player has to chose one of three characters to oppose it. Wild monsters attack elves and humans, with the one behind this menace being known as the Black Druid. The player has to face many different creatures before the Black Druid is located and killed. It turns out that the true enemy are Vagar, a race of corrupted beings of whom Black Druid was a member. They serve their mistress, Malith, once a leader of Agallan giants, now a fallen creature who takes its powers from Throne of Agony. The hero has to defeat Vagar, find the Agallan giants, and ultimately defeat Malith. Finally, the player has to decide what to do with the Throne of Agony, a sentient throne of darkness, as it could be either used, or destroyed.
Source: Wikipedia, "Dungeon Siege: Throne of Agony", available under the CC-BY-SA License.