Aku is a being of pure evil and the nemisis of Samurai Jack. He started his existance as a roiling cloud of darkness haunting the universe. The the gods Odin, Ra, and Vishnu confronted him. A small bit of him survived the battle and hurtled through space, eventually impacting on primative Earth as the asteroid that exterminated the dinosaurs. Throughout time he plagued the inhabitants of Earth as a pool of black sludge and spiked pillars of darkness. Eventually an emperor of Japan rode to confront him with a magic potion. Rather than destroying him, the potion freed him. He has the power of shap-shifting, but his prefered form is a towering figure of darkness with a mouith full of fangs and flames for eyebrows. The emperor defeated him an imprisoned him in the earth, but he was released many years later during a solar eclipse. Samurai Jack fought him but before Jack could make a killing blow, Aku used his powers to send Jack into the future. In the interveneing years, Aku conqured the Earth and populated it with aliens, robots and supernatural beings. He is immune to conventional attacks, but can be harmed by magical ones, particularly Jack's sword, or the powers of gods or supernatural beings. He occasionally fights Jack himself, but Jack always gets the better of the fight. Aku alwyas manages to escape with the threat of getting Jack next time.