Twelve-Winged Dark Seraphim
I think the Knife King didn't do what they think he did. His life for shorter? I'm guessing he made a wish to be strong enough or that he wished Armor God left (or something like that) and his life was taken. That said, it seems a little odd that the scabbard was healing the space... maybe it's a container for knife skills and it builds up or compounds knife skills till the time you decide to release them.... that would be awesome đ