Sophia Dryden was possessed by a demon during the final battle that took place during of her attempt to steal the crown. The demon has been locked up in the keep since then and wishes to roam the rest of the world, "feeding". She asks the Warden to free her by killing all in the keep's tower. In exchange, she offers to seal the rift into the Fade to prevent other demons from coming through. If pressed, she promises to reveal the hiding place for 8.
Enter the keep and reach Sophia in the Commander's Quarters on the second floor. If the Warden accepts her offer, the quest will be added to the journal. Leave the second floor, cross the bridge (with traps and skeletons) and enter Avernus's Tower to find the mage Avernus. Once he is defeated, return to Sophia.
Alternatively, the Warden can side with Avernus and return to the main body of the keep with him. Sophia will be waiting and a fight to the death will follow. The quest will be completed when either Avernus or Sophia are killed.
See Meeting Sophia Dryden for more information (and clues to party reactions) when dealing with Sophia.
Loot, depending on which of the two antagonists the Warden sided with.