Kill Mug'Thol, and return his head with the Crown of Will to Melisara in Tarren Mill.
With Mug'Thol's lieutenants dead, all that remains is to kill Mug'Thol and retrieve the Crown of Will. I have no doubt he'll be reluctant to part with it. Most likely expecting you, too, given the... departure... of his fellows.
Find Mug'Thol, and deliver his head and the Crown to me. The Crushridge resistance will be at an end.