Kil'jaeden

From wow.incgamers.com

Eredar Demon Lord. Commabder of the Dreadlords. Apprentice to Sargeras. He contacted Gul'dan across the Twisting Nether and assisted the young Warlock in becoming very powerful as well as enabling the Orcish Horde to invade Azeroth.

He has recently begun attempts to invade Azeroth once again, through the means of Prince Kael'thas Sunstrider and the Sunwell on Isle of Quel'Danas