Ancestor Spirit is an optional boss, to gain access to him you need to light 8 fires ny the stairs of Hallowhorn Grounds. Once you have done this the corpse of the Ancestor Spirit will glow and you can interact and enter the arena.
I found this boss to be quite easy, many of his moves are easily telegraphed. He will do frost attacks, horn attacks and kicks with his feet.
There are occasions where he does heal himself, I suggest running in and smacking him to reduce the amount he heals.