Aku

Long ago in the vastness of space, a great, formless evil called the Ultimate Evil arose. Before it could do harm to the universe, it was set upon by the three prime deities of the Aesir, Egyptian, and Hindu mythologies: Odin the All-father, Ra the Sun God, and Vishnu, the Supreme God and Soul. The three deities fought a fierce battle against the evil manifestation, burning the entity almost completely out of existence with their divine power. However, during the battle, one small fragment of the entity that was cut from the whole by Odin did not burn away. Unnoticed by the three deities, this fragment floated away from the site of the battle and drifted through space for an untold amount of time, until it eventually crashed to Earth during the Cretaceous period of the Mesozoic era. It is implied that this crash is the event that caused the extinction of the none-avian dinosaurs, although the crash occurred in ancient Japan as opposed to the Chicxulub impactor, which touched ground in modern day Mexico, so this may not be the case. The fragment developed over eons into an ominous forest of black spikes that devoured any who dared enter. Eventually the forest grew so large that The Emperor (Jack's father) decided to kill the evil at its source. Armed with a magic oil given to him by Buddhist monks, the Emperor and his cavalry rode into the heart of the forest. But as they rode, the forest began to kill his men, leaving the Emperor himself as the only survivor. Once at the black lake in the forest's center, the Samurai Lord doused an arrow into the oil he was given and fired it into the black lake, believing that it would destroy the evil. Unexpectedly, the oil's magic had the unforeseen effect of galvanising the evil into a human-like form, giving it sentience and shape-shifting abilities, and thus giving birth to the demon wizard, Aku.

The Emperor attacked Aku, but, with nothing more than his mundane, mortal weapons, he could inflict no harm upon the monster. Aku easily defeated the Emperor, and trapped him to the side of a tree so that he could watch as Aku destroyed his home and killed his people. However, the Deities, sensing the good in the Samurai Lord, summoned him to them. Taking the good inside him, they used avatar bodies to create an enchanted sword that was capable of harming Aku. Armed with the sword and a magical suit of armor, the Emperor plunged into battle against Aku. In the end, the Emperor was victorious, but he was unable to destroy the demon, and so instead sealed him in the devastated wasteland in the form of a jagged black tree. As he was sealed away, Aku vowed he would one day return. During the battle, the Emperor's wife gave birth to a son. Determined to learn from the catastrophe, and aware that Aku would indeed one day return, the Emperor and his family hatched a plan, and orchestrated the events that would set Jack on his eventual years of training.