This is a
secret bossfight and necessary for getting the true ending.
The fight is in
The Void and requires you to gather the
3 demon keys.
You probably passed the locked door as it's early in the void, finishing that mini dungeon has this as the fight at the end.
I would recommend avoiding the fight until you get Normal End, then simply don't save so you can go back and do this fight, beat the final boss again and you'll get the True End as well. Both achievements in one playthrough.
I go into more detail on the steps in my guide for The Seed of Life achievement: