To unlock the secret boss fight you need two items, the Hasabe Charm and the Mami Charm.
The Hasabe Charm is unlocked after beating Sabuko when you finish the game, the Mami Charm is unlocked after you destroy all 25 statues.
Once you have both items, equip them, and return to the room where you fought Sabuko. This will trigger the secret boss fight to start. It's not too difficult, just have some food on you in case you get into trouble. Just be ready for a fourth phase just like the Sabuko fight, and make sure you attack the Mami angel during this phase to get through it quickly.