Aku (Samurai Jack)

Long ago—very long ago—a single small fragment of an Eldritch Abomination fell to Earth, wiping out the large saurian creatures that dominated the planet at that time. It existed as a mindless evil for millions of years until a brave king attempted to destroy it: the attempt to destroy it instead gave it consciousness. It became Aku, a being of pure darkness and evil. When a noble samurai came to fight him, Aku tore open a portal in time and flung him into the future, where he is de facto ruler of the world. Now, the samurai called Jack has drawn every drop of his ire, and he devotes a lot of effort to finishing him once and for all.