Hugh, especially during his fight against you. You can't imagine when you are being mind-controlled by Dracula and not knowing what the hell you are doing until it is too late.
Legion's more well known design in Castlevania—a giant ball of human bodies—is already fairly creepy, but in this game, it is a mass of sickly purple heads with tentacles sticking out of it. Fans who have fought Dracula in Dracula's Curse might find this version of Legion familiar as well.