Aku

"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!"

Aku is the chief antagonist of the popular cartoon series Samurai Jack and is depicted as a demonic creature of near-infinite power and malice whose only real aim in existence seems to be spreading evil, in fact his name means "evil" in Japanese so he is very likely a living embodiment of such a force.

Originating long ago in the depths of space as a formless evil Aku was banished by the gods Odin, Rah and Vishnu in order to protect the universe from his influence - yet a small shard of the great shadow managed to survive and flew off into space - for many years this shard continued flying through space until it eventually landed on Earth, causing the extinction of the dinosaurs in the process.

Many years later the site on which Aku landed would become the ancient island of Japan, which Aku infected like a horrible disease - creating vast forests of evil in the land and threatening to consume it, eventually Aku's threat became so great that Jack's father decided to rid the land of the demon once and for all using magical-oil made by Buddhist monks, however when he fired an arrow coated with this oil into the evil entity it didn't destroy it but rather gave it form and sentience, thus forming Aku as a humanoid threat of unstoppable power.

However with aid from Odin, Ra and Vishnu Jack's father created a magical-sword powerful enough to defeat Aku and fought the demon, at the end of the conflict Aku was transformed into a tree - yet the imprisonment wouldn't last much more than a decade before a solar eclipse freed Aku again.

Aku would then proceed to lay waste to the kingdom Jack's father ruled and engaged in a battle with Jack himself - Jack very nearly killed Aku for good using the magical-sword but Aku threw open a portal in time and transported Jack to a future-world in which Aku ruled supreme.

For the rest of the series Aku is portrayed as the all-powerful ruler of the world and is an almost unstoppable force - yet he fears Jack, knowing fine well the warrior has the power to slay him, for this reason Aku has placed a high bounty on Jack's head and is forever trying to dispose of him, whether it be with manipulation, force or trickery.