Aku

"What's this? It appears the Preschool Girls and I are being secretly monitored! But by who? Oh, no! Not... I--I--I can't say it! This is a villain so evil, so sinister, so horribly vile that even the utterance of his name strikes fear into the hearts of men! It's Aku."

--Su Ji-Hoon, The Peace Treaty

Aku was the main antagonist of the series Samurai Jack. Born from a cosmic black mass of darkness, Aku was an evil, shapeshifting demon that has taken over the world and ruled it for countless eons. Throughout the series, he was voiced by Mako Iwamatsu in Samurai Jack's seasons 1-4, but due to his death in 2006, he was replaced by Greg Baldwin in Season 5 of Samurai Jack.