Gorgoroth was an Unborn Ogre created by Oryx to serve as a vessel for the Deep. We don't have any other concrete details, but it seems that a couple of the worm gods were involved with the creation of Golgoroth in some way or another. There is also no mention of Golgoroth being reborn, so it is possible that the Darkness using it as a vessel mutated it beyond a normal Ogre's capacity.