Emperor of Darkness Trophy in Bloodstained: Curse of the Moon

To unlock this ending, you must kill/sacrifice Mirim, Alfred and Gebel when you encounter them. Then beat the game with Zangetsu alone, with all his soul arts.

When you get to the end of the game, and defeat Gremory, you find that she charges her moon attack and Zangetsu just stands in the middle, looking up. Get to the center and hold to charge then release when fully charged to finish her off.

Beating this mode will unlock Ultimate mode and Boss Rush mode.

You can also do this on Normal Mode, if the bosses are too difficult in Ultimate. Note that you won't have any Soul Arts and cannot pick up any upgrades if you do that method.