Kill Marl Wormthorn at Maldraz in the Blasted Lands. You can use the Demoniac Vessel to pull him out of his trance.
When word gets out about what we've done... there's no saying what the citizens of Surwich will do. They cling so closely to Wormthorn's dream...
...but still, it must be done. Take the demoniac vessel and use it on Marl Wormthorn to pull him from his trance. Return to me when he is no more.