Aku is a fictional character and the main antagonist of the animated series Samurai Jack. He is a ruthless, diabolical, and manipulative Japanese demon king who self-proclaims himself to be the "Shapeshifting Master of Darkness" and is the archenemy of the eponymous hero, Samurai Jack.
Eons ago, Aku was merely a large black mass of pure evil that appeared out of nothing and nowhere, but was attacked and vanquished by a triumvirate of gods: Odin, Ra, and Vishnu as a means to protect the universe from its influence. Unfortunately, a small shard of the great entity that would become Aku was able to survive and drifted off into space, which throughout the eons continued to tumble through space until landing on Earth and caused the extinction of the dinosaurs.
Years later, the site of which the shard landed that would become the ancient island of Japan, which infected like a horrible disease, creating vast forests of evil throughout the land and threatening to consume it all. Eventually, the shard's threat became so great and impressive that the Emperor of a Japanese kingdom decided to rid the land of the evil substance once and for all by using a magical oil made by Buddhist monks. Unfortunately, when he fired an arrow coated with this oil into the entity, it did not destroy it, but rather gave it form and sentience, forming a humanoid threat of unimaginable power named Aku. No mortal weapon could harm him but, with aid from Odin, Ra, and Vishnu, the Emperor wielded a magical sword with the ability to harm Aku.
