Aku sends out Demongo, the soul collector, to destroy Jack. Now Jack must fight all the mighty warrior souls that Demongo has collected.
EP 12 XXXVIII
When ancient gods try to destroy a powerful evil, they send one piece of the beast hurtling through space. This piece lands on earth, spreading its evil through the land. When it encroaches on an Emperor's empire, he takes it upon himself to destroy the evil, but ends up unleashing the evil that is Aku.