Gul'Dan

From wow.incgamers.com

Orc Warlock. Taught by the Eredar Demon Lord Kil'jaeden. Formed the Shadow Council, which took control of the inner workings of the Orcish Horde, intensifying it's hunger for battle. Appointed Blackhand the Destroyer Warchief after the Orc's defeat at Stormwind Keep.

During the ongoing battles between the Horde and Humans Medivh was fatally attacked by a band of warriors. Gul'dan, sensing Medivh was close to death, entered his body to extract the location of Demonlord Sargeras' tomb. However Medivh died before he could do so, unleashing a backlash of power that sent Gul'dan into a deep sleep. His fellow Warlock followers guarded his comatose body.

When he awoke after a long sleep he was not pleased to see Orgrim Doomhammer in charge. Doomhammer took Gul'dan's citadel near Stormwind Keep and he executed all members of the Shadow Council apart from Gul'dan as he feigned allegiance to Doomhammer.

Whilst Doomhammer was off conquering Quel'thalas, Gul'dan and his Stormweaver Clan, accompanied by Cho'gall and the Twilight's Hammer Clan set off to find the tomb of Daemonlord Sagaeras. When they found it they set the Lord of the Burning Legion free. In retaliation of this betrayal Doomhammer ordered the destruction of the Stormweaver Clan and his warriors returned with the head of Gul'dan.