The real William was the king of England 1066-1087. He caused the Battle of Hastings in 1066 and defeated Harold Goldwinson. In HTF world, he is a brutal killer who is just as bad as Al Capone. Infact, both are friends in the HTF world despite being far away in real time.
The real William died in 1087 when he fell off his horse and suffered abdominal injuries.