Collect Vim'gol's Vile Grimoire.
There is one more gronn to be slain: Skulloc Soulgrinder. He keeps himself hidden away, but there is a way to coax him forth.
He was trained by an ogre warlock known as Vim'gol the Vile, whose grimoire you must retrieve!
Vim'gol must be summoned by standing within a fire ring at his circle. Find Vim'gol's Circle in the mountains, to the northeast, near the Broken Wilds.