Klass Metalfist was born 18 years before the First War. He was a willing receptacle for the arcane influence of the demonic powers, and joined the Shadow Council to prepare for the first invasion of Azeroth.
He was a junior warlock, and thus was not destroyed after the Second War. He helped pull together the remaining warlocks and followed Thrall and the Horde across the seas to Kalimdor. The warlocks attempted to aid the Burning Legion, even tricking some orcs into accepting demonic influence for more power, but still fled, defeated, at the end of the war.
Klass helped build Thunder Axe Fortress in Desolace, allying himself with the Magram centaur in order to keep his hideout secret. He performs dark magic in the fortress, but leaves the demon-summoning to Al'arr, as she is even more secluded than he. He wants nothing less than the usurpation of Thrall and to become the leader of the Horde. He also wants to topple the Alliance, and initiate the return of the Burning Legion.