The Githyanki have set up a trap in Ember for you. You have to hold off two waves of enemies. They are numerous, so remember to use high-range spells. When you're done, talk to Bishop. You'll loose influence on him if you show any compassion to the locals. A woman named Alaine will appear and tell you where the Giths have taken Shandra. Before you leave, talk to a boy named Marcus. He'll want Bishop's knife from you. Agree of refuse, it doesn't matter. This subject will return sooner or later.