"Gorefiend, Gaz Soulripper, and the other death knights swore allegiance to Doomhammer and fought for the Horde where their powerful energies aided in many battles. When Gul'dan betrayed the Horde, Gorefiend remained in the service of the Warchief until he was defeated. Leading the death knights beyond the Dark Portal before its destruction, Gorefiend remained on Draenor until Ner'zhul, Gul'dan's former master, came to the forefront and united the remaining clans into the Horde of Draenor. Ner'zhul intended to open new dimensional gateways to other worlds, but needed powerful items to stabilize them. To that end, Ner'zhul sent Gorefiend through a reconstructed Dark Portal to claim two powerful artifacts required for his spell: The Book of Medivh and the Eye of Dalaran." - From WoWWiki.