"Long ago, in a distant land, I, Aku, the shape-shifting Master of Darkness, unleashed an unspeakable evil! But a foolish Samurai warrior wielding a magic sword stepped forth to oppose me. Before the final blow was struck, I tore open a portal in time and flung him into the future, where my evil is law! Now the fool seeks to return to the past, and undo the future that is Aku!"-- from the show's opening (seasons 1-4).
He was a primordial black mass eons ago. He was separated into pieces by Odin, Ra and Rama. A tuft of him was sent to Earth, killing the dinosaurs. In feudal Japan, the Emperor was about to destroy him with a poisoned arrow and sun energy. The black mass became a tree-like creature and gained arms, hands, a face and a voice. The black mass was known as Aku, the Shogun of Sorrow and the Deliverer of Darkness. He almost destroyed Feudal Japan. He met the Emperor again but the Emperor had a sword of righteousness made by Odin, Ra and Rama. The Emperor used it to attack Aku. Aku shapeshifted after various attacks. He got separated and infected samurai armor and became an army of Samurai Akus. The Emperor killed every last one and stabbed Aku's mass into the sword and was sent into the Earth. Aku became a tree for the next ten years. He reborn by an eclipse. He ravaged Japan yet again. Aku made the Japanese his slaves. The Emperor's son was about to kill Aku but Aku sent the Emperor's son to the future. The Emperor's son took the name Jack after a couple of teenage aliens called him Jack. Aku sent an army of robotic beetles to defeat Jack. Jack like his father, killed every last beetle. Aku wanted to prevent Jack from going back to Feudal Japan.
Fifty years have passed since Aku made Jack fail at every attempt to get him back to the past and destroyed every time portal in the process. He met a High Priestess and made her give birth to seven daughters from his own essence. He became bored, not remembering the fact that he fathered seven daughters. His ally, Scaramouche the Merciless informed Aku that Samurai Jack lost his sword. Once Jack finally encounters Aku with his surviving daughter Ashi, he saw Jack got his sword back and destroyed Scaramouche.
He let Ashi unleash her darkness and imprision Jack. Jack let Ashi time travel to the day when Aku opened a portal for Jack. He was surprised by Jack's arrival and finally met his demise at the hands of the samurai, at the cost of losing his daughter.