Treasures of the Sun is downloadable content (DLC) for Dungeon Siege III. It adds new content that supplements the main quest. You get to travel to an all-new environment: the Aranoi Desert with an all new quest line, characters, items, monsters, and dungeons to explore. The DLC also has features that can be used in the main game including enchanting and three new ultimate abilities. The quest of Treasures of the Sun brings you to an abbey in the middle of the desert to solve the mystery of the greatest treasure of the Azunite faith.
The DLC is accessed from the main story path (in the Stonebridge Chapterhouse or before the final boss area), and can take around 3-5 hours to complete.