Arbaal the Undefeated is one of the original Chaos Champions from the earliest editions of Warhammer Fantasy Battles. A sworn servant of Khorne, he is perhaps the greatest champion of all those dedicated to the Blood God; thousands have fallen before his axe, and none have ever conquered him. As signs of favor, Khorne gifts Arbaal with the Hound of Khorne, a monstrous Flesh Hound that serves as a steed, and infuses him with tireless energy for the slaughter, but should Arbaal ever flee from battle, then Khorne's wrath will be unsurpassed.
In crunch-terms, he rode into battle upon a mega-nasty steed that was immune to magical weapon attacks and made them both immune to spells; he traded the usual Frenzy rules of the time for Destroyer of Khorne (2D6 Strength 6 Weapon Skill 9 attacks each round), and if he ever fled combat he would instantly turn into a Chaos Spawn. He only came with chaos armor and an axe, but this was the edition when special characters could buy extra gear to pimp themselves out.
Arbaal was rumored to return as the Khornate Lieutenant of Archaon as part of the next release for The End Times; while he does indeed return, he gets killed by Caradryan, Incarnte of Fire. So much for the Undefeated.