Anub'arak is a nerubian lord

He was actually originally king. When the scourge's plague spread nerubians were immune, so the undead masses/dreadlords had to phyiscally kill them.

When king Anubarak was killed, he was stripped of his title when rezzed as the Lich King's servant. His new job was to be the Frozen Throne's first line of defense in Northrend. But he still maintained his own personal will at this time, he was just ... "mindcontrolled?" He's actually thankful to the heros for killing him in WoW WoTLK.

He is then rezzed a 2nd time, completely scourged and dominated, all will and free thought striped from him.