The Scarlet Crusade is on the move. Somewhere along the road from Tyr's Hand to Stratholme you will find the Oracle, Demetria, and her entourage. Hunt her down and slay her. Return to Nathanos Blightcaller should you succeed.
Demetria is chief advisor to the Grand Crusader. She is called the Oracle due to her 'otherworldly' senses. She has successfully assisted the Crusade in several of their victories against the Scourge.
While this ruse was successful in exposing her, if she truly does possess psychic powers, she will undoubtedly be expecting some sort of trouble. You are going to provide that 'trouble.'
Track down the Oracle and terminate her. They should be moving out of Tyr's Hand as we speak. Make me proud, worm.