I think the whole deal is that i's an amalgam of every being that's ever linked the fire.
Meaning the Chosen Undead, the Bearer of the Curse, AKA every player character ever, is a part of it. As it has all that in it, that's why it has such a diverse move set in it's first form.

Hell, assuming you saved him, Solaire is a part of it.

Any final boss will always be brilliant with good, dramatic piano music