Moon Presence is an optional end boss in Bloodborne. You unlock this boss by consuming 3 of the 1/3 Umbilical Chords, these can be collected at various points in the game. Luckily this boss ends up being kind of easy, which was especially nice considering you must fight him immediately following Gehrman, who is not so easy. If you plan on going for this ending make sure you have your highest Communion Rune equipped, those extra vials are vital, trust me. Good luck hunters, let us transcend this mortal husk and become as Gods!
Bloodborne Bosses: Moon Presence Strategy