Teron Gorefiend

From wow.incgamers.com

Teron Gorefiend was once a powerful Warlock of the Shadow Council, but [[Orgrim Doomhammer|Doomhammer]'s terrible gaze fell upon him after his ascension, and, along with most of his brethren, Gorefiend was slain. However, his haunted spirit was only too willing to come to the beckons of Gul'dan when he created his Death Knights. Gorefiend's tormented spirit was instilled within the corpse of a once-noble Knight of Azeroth. Gorefiend served Gul'dan well, but silently, and so it was easy for him to switch allegiances when Gul'dan and his followers were utterly slain at the Tomb of Sargeras. He remained hidden with the Bleeding Hollow Clan until their crossing over into Draenor. The always self-serving Gorefiend had discerned many of the secrets of the Dark Portal during his time in the Second War. He only leant Ner'zhul his dark services if the Shaman pledged to allow Gorefiend to claim a world for his own when the rifts were opened. So, after assisting the Shadow Moon Clan in acquiring the items he would need for the vile ritual, Gorefiend returned to Draenor, helped Ner'zhul to open the rifts into the Twisting Nether, and fled the dying world into the rifts.

In the end he was defeated and imprisoned in what now is Shadowmoon Valley, where an unthoughtful hero set him free without knowing it. He proceeded by travelling to the Black Temple, and offering his services to Illidan Stormrage.