Destroy Blastshadow the Brutemaster's soulstone.
This nightmare will never be over while Blastshadow the Brutemaster still breathes. There are a couple of problems though.
One, being a warlock, he's got a succubus named Delicia Whipsnaps. If you kill Blastshadow quickly, the demon will go back to where it was summoned from.
The second problem, since Blastshadow is a warlock, he has a soulstone that allows him to resurrect. You have to get his soulstone and destroy it so that he'll be dead forever!