Abhorash

The most legendary of all Blood Dragons was and is their first master, the Dragon Slayer, the Scourge of the Greenskins, the Blade Immortal, the Lord of Blood, Abhorash the Great, Abhorash the Wanderer, Abhorash the Master. But his location is unknown. Some say he wandered north, deep into the Chaos Wastes, or east into the land of the Giants to seek out even greater conquests. Others say he followed the path of Sigmar and became a God; others say he still walks amongst their ranks in the disguise of a young thrall, watching for the most worthy amongst them. Still others think of the tales of other Dragon slayers, like Gilles de Breton and Lord Amara of Hoeth, and wonder how many faces their master might wear in a life as long as his. Whatever the case, none have seen or spoken to him in centuries, and his location and his plans remain mysterious.

Source

 * Warhammer Fantasy RPG 2nd ED -- Night's Dark Masters (pg. 49).