Go to Bane Blackblood's Castle and find Dwarthon Stoneface (west of Bane's Throne).
Say Bane to him and he will instruct you to kill Delwobble the Mad Summoner who is located in the Castle's Dungeon.
Find Delwobble (he is at the far east, in a small corridor for which the only access is the northeast corner) and kill him.
Return to Dwarthon and say Bane.
Quest Reward Synopsis : You can talk to Bane Blackblood