Kil'jaeden

• Also known as Kil'jaeden the Deceiver.
• An Eredar warlock in command of the Nathrezim, in the service of Sargeras.

From WorldofWarcraft.com:

Kil'jaeden the Deceiver was chosen to seek out the darkest races in the universe and recruit them into Sargeras' ranks.

also:

Kil'jaeden's first move was to enslave the vampiric dreadlords under his terrible power. The dreadlords served as his personal agents throughout the universe, and they took pleasure in locating primitive races for their master to corrupt and bring into the fold. First amongst the dreadlords was Tichondrius the Darkener. Tichondrius served Kil'jaeden as the perfect soldier and agreed to bring Sargeras' burning will to all the dark corners of the universe.

and:

Around the time of Medivh's birth on Azeroth, Kil'jaeden the Deceiver sat and brooded amongst his followers within the Twisting Nether. The cunning demonlord, under orders of his master, Sargeras, was plotting the Burning Legion's second invasion of Azeroth. This time he would not allow any mistakes. Kil'jaeden surmised that he needed a new force to weaken Azeroth's defenses before the Legion even set foot upon the world. If the mortal races, such as the Night Elves and Dragons, were forced to contend with a new threat, they would be too weak to pose any real resistance when the Legion's true invasion arrived.

It was at this time that Kil'jaeden discovered the lush world of Draenor floating peacefully within the Great Dark Beyond. Home to the shamanistic, clan-based Orcs and the peaceful Draenei, Draenor was as idyllic as it was vast. The noble orc clans roamed the open prairies and hunted for sport, while the inquisitive Draenei built crude cities within the world's towering cliffs and peaks. Kil'jaeden knew that Draenor's denizens had great potential to serve the Burning Legion if they could be cultivated properly.

Of the two races, Kil'jaeden saw that the warrior orcs were more susceptible to the Legion's corruption. He enthralled the elder orc shaman, Ner'zhul, in much the same way that Sargeras brought Queen Azshara under his control in ages past. Using the cunning shaman as his conduit, the demon spread battle lust and savagery throughout the orc clans. Before long, the spiritual race was transformed into a bloodthirsty people. Kil'jaeden then urged Ner'zhul and his people to take the last step: to give themselves over entirely to the pursuit of death and war.

See also:

• The Shadow Council
• The Jaedenar

From WorldofWarcraft.com:

• The Rise of the Horde
• The Birth of the Lich King
• Icecrown and the Frozen Throne
• The Betrayer Ascendant