You must retrieve Vim'gol's Vile Grimoire. Deliver it to Mog'dorg the Wizened atop the tower at the Circle of Blood in the Blade's Edge Mountains.
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 yourself, and four others, each standing within a different fire ring at his circle. Find Vim'gol's Circle in the mountains, to the northeast, near the Broken Wilds.
And, [player], if you hear Vim'gol talk about growing bigger, get back to the fire rings!