I do not enjoy associating with the likes of those who meddle in the powers that my forsaken brother does. But for this, it is necessary.
He helped us before during the Black Rebellion, but I believe our Alliance between him has all but withered. You will need to bring something to bargain with. Speak with Malathil Gravefall in Tirisfal Glades, near Brill. Ask for a trap that could bind even the greatest demons to this reality.