Bohemond Beastslayer is one of the fairly large assortment of special characters for Bretonnia that appeared only in Warhammer 5th edition and were subsequently removed when it received its more grimdark revamp in 6th edition.
The current Duke of Bastonne, Bohemond is actually a direct descendant of King Giles le Breton, and thusly could challenge King Louen Leoncoeur for the right to rule Bretonnia. However, he's quite content with his lot in life and totally loyal, so he has no inclination to do that. Even by Bretonnian standards, he's absolutely bonkers for fighting worthy opponents; as his common moniker suggests he basically loves to go hunting monsters across Bretonnia, wielding the deadly Beast Mace of Bastonne, a dragon's thighbone topped with a lump of meteoric iron, as he seeks monstrous skulls to bash. Truth be told, he really doesn't like ruling much... unfortunately, he's a terrible judge of character, and so invariably has to kill and replace his appointed stewards and justicars when they turn out to be corrupt (in in the mundane sense or in the sense of being Chaos worshippers, vampires, etc). This just drags him further away from his beloved hunting, which he resents.