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Nightmare Fuel: Stupid Mario Brothers
The Darkness, BIG TIME. While he's not the first "serious" villain the show had, he is definitley the most vicious, ruthless, and downright monstrous of them all! He's the reason Mario and the Gang almost lost in Act 2 of the Movie. He can appear out of nowhere and can't be killed without a spell. And even THAT didn't keep him down for long!