Zeromus the final boss of the game. To make it reveal it's true form, Cecil must use the Crystal and he's the only one to do it.

Once Zeromus gained his true form, now it may be your chance to cast slow on it and steal the "Dark Matter" from him as well.

To be warned, Zeromus's 'Big Bang' is highly destructive that it will surely KO most of your party and a additional effect 'Sap', the most annoying status effect. His 'Black Hole' spell nullifies all magical effects (such as slow, protect, haste, shell, sap, etc.). And he can also use the powerful Black Magic 'Meteor'.

This boss battle is a pain but your party must be around LV 70-80 to beat it.
Good luck