I added the name of his patron (Ur-Gothoz) onto the Scroll of Retribution and, after what feels like a couple of weeks in game, he told me that his blackguard power is gone. Poison Weapon has been nerfed and if I really need dispel magic on hit I still have Staff of the Magi so I thought it should be okay to set him free. But the problem is, he started to act really strange after the whole event, it feels like playing a online game and he *lags*, keeps falling far behind everyone even with the Paws of Cheetah AND Silver Dragon Scale, and does not follow a moving enemy in combat.
Edit: He only lags in the area where the dialogue is triggered. I managed to trigger that dialogue right outside Watcher's Keep to minimize the damage so the problem is kinda solved, but it still feels strange.