Shistal was a former monk of Candlekeep who was killed at some point by one of Sarevok's lackeys so a Doppelganger could take his form and infiltrate Candlekeep.

Before you see Shistal, Bendalis will warn you that he has not been acting like his usual self. When you speak with him he will act as if he never knew you and if you keep confronting him about his lack of knowledge about his past life or how he knows you have been to Cloakwood, he will turn into a Greater Doppelganger and attack.