See Magic Sympathizer for more details about this quest.
Once you escape the Fade, then you have to face the blood mage that has overtaken the Tower. You'll meet an imprisoned Templar on the way up to the blood mage. He'll ask you to kill all of the Mages. Agree with him and kill them after the fight with the blood mage.
Once the fight is done and you have killed all of the Mages, return to the first floor of the Tower and say that the Tower is secure. The Templars will join your army as a result.