"Episode XXXVII: The Birth of Evil, Part 1" is the thirty-seventh episode in the third season of Samurai Jack. This two part episode tells the story of how Aku came to exist, and battled against Jack's father.
Synopsis
In the depths of space, a giant black mass of emptiness appears and threatens to consume the entire universe. Three ancient gods Odin (King of the Norse Gods), Horus (the Sun God of what would become Ancient Egypt) and Vishnu (one of the three supreme dieties of Hinduism) arrive on the scene and pursue the mass. After a fierce battle, the three Gods succeed in destroying the mass. However, one fragment survives and drifts away into the cosmos.
After an unknown period of time, the fragment eventually finds its way to Earth. The fragment crash lands on Earth in what would later become the island of Japan, leaving an impact that brings about the extinction of the dinosaurs and, in its wake, leaves behind a giant pool of a black, tar-like substance that grows and expands as time passes spewing forests of giant, black spikes. As life on Earth begins to evolve, many people and animals fall victim to the deadly pit.
By the time Japan's monarchy is established, the pool's invasion, along with the forests that have started to impale homes and buildings, has grown so out of control that the current Emperor (whom would later become Jack's father) decides he must destroy the threat at its core. Despite the desperate pleas from his distraught, pregnant wife, the Emperor tells her that while she is the most important thing to him the people of the land are also his family and that if doesn't act the entire land will perish. After recieving an elixir of poison believed to be the key to putting an end to the tar pool once and for all, the Emperor, along with an army of soldiers, rides into the forest toward the center where the heart of the pool lies. Before long, more spikes start to errupt from the ground killing all of the Emperor's men, but he rides on.
Eventually, the Emperor reaches the center of the forest. He takes an arrow and laces it with the poison. After saying a prayer, he fires the arrow into the sky where it catches fire from the sun before landing directly into the center of the pit. At first the plan seems to have worked, as the spikes start to retreat back into the pool. However, it is soon revealed that the plan has backfired completely: instead of destroying the pool of blackness, the elixir gave it life. Soon a massive creature with giant horns, flaming eye brows, giant teeth, and a body made from a black void of emptiness emerges from the pit. Aku (Japanese for "Evil") has entered our world.
When Aku notices the Emperor on the ground he thanks him for releasing him. However, when the Emperor tells him he was tring to destory him, the evil shapeshifter laughs mockingly at his failed attempt. The Emperor tries to kill Aku using his remaining arrows but his attempt proves futile as he is immune from any damage caused by mortal weapons. As punishment for his insolence, Aku traps the Emperor on a giant rock in the pool of emptiness and then sets his sights on the Emperor's kingdom. Aku descends on the Emperor's home land and begins to destroy it, as the distraught ruler is forced to watch helpless.