I remember that i once saw an anime (pretty short-like 10-15 ep).. it was about a guy who had to fight different people to reach his goal. At a moment he fought one guy- a knight who had a creature in his belly (like a big worm) that didnt let him lose any battle/die, After that the knight told the main character how to help him kill that worm, but he couldnt live without it so he died.