Because what group about vile villains in saccharine shows wouldn't have a thread involving this creepy dude:

Meet The Lich (voiced by Ron Perlman). Before he was introduced, Adventure Time mostly revolved around the wacky adventures of Finn and Jake and the closest thing to the main antagonist up to that point was the Ice King, who was a total goofball. Then this ancient evil showed up. A byproduct of the Mushroom War and the last scholar of GOLB, The Lich desired death and destruction across the universe. So when people say The Lich is not funny, they mean it. In fact, the only times he is funny are very rare:

Oh, yeah. He straight-up murdered a character on-screen:

The Lich is without a doubt one of the scariest antagonists in cartoon history.

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Indeed he is. And given how there are plenty of cartoons that have dark and scary villains, that's no easy feat.