Quest from: Sir Nevalle;
You'll be taken to the city watch's HQ, where Aldanon and Brelaina are discussing the matter of a githzerai prisoner named Zhjaeve. She's the next NPC to join your team. Take her with you and go back to the Keep. Apparently Lord Nasher decided to give you the honor of managing it - you have to take care of its development so that it's back to its former glory. Check out the appropriate guide section for details on Crossroads Keep. You'll also have a long chat with Zhjaeve, who'll point you towards your next goal: the Ritual of Purification.
HINT: Now is a very good moment to take care of numerous side-quests you've encountered on your journeys. Take your time to finish all the loose ends, such as NPC's quests (Khelgar), collecting employees for the Keep (especially Orlen from West Harbor, who'll become unavailable once you take the main plot one step further), and raise some experience.