The Black Mass is a living, non-sapient mass of pure evil from outer space. It is arguably the true antagonist of Samurai Jack, because it is the source of all evil. Before it could consume the universe, three deities (Odin, Ra, and Vishnu) arrived to exterminate this great evil.
The Black Mass appeared in "Episode XXXVII: The Birth of Evil, Part 1" when it attacked the universe. Odin, Ra and Vishnu arrived to pursue the mass. After a fierce battle, the three Gods finally defeated the Black Mass. Unfortunately, it was not completely destroyed: a fragment of the black mass drifted away from the battle unobserved and crashed into the Earth in the year 65,000,000 BC, killing the dinosaurs.
As life on Earth began to evolve anew, many people and animals are destroyed by the mass as it continually expanded. When the creature approached Imperial Japan, Samurai Jack's father, who was the Emperor, attempted to destroy the Black Mass with a poisoned arrow, which only resulted in the Black Mass becoming sapient and aware. Taking the name Aku, the creature went on to eventually control the Earth.