The Japanese knew him as Amatsu-Mikaboshi, the shape-shifting god of evil but in the Marvel Comics, Mikaboshi was a primordial entity that existed before there was any actual world whose battles with the emerging Amatsu-Kami entities would become the building blocks for the Shinto religion's mythology. His desire to return the world to a void of nothingness would bring him into conflict with various gods and goddesses of other mythologies as the self-described Chaos King would come close to accomplishing his apocalyptic goals.
In the beginning, Mikaboshi's dark void was threatened by the spirit of Gaea and the Amatsu-Kami, humanoid entities hailing from a nearby pocket dimension who began to shape the Earth and populate it with the first humans. When the Amatsu-Kami began to be worshiped as gods, the enraged Mikaboshi attacked the developing pantheon and upon his defeat, he was imprisoned within Yami, the Japanese realm of the dead where he remained for centuries.
His official debut was in the Thor: Blood Oath mini-series where Thor and the Warriors Three had to descend within Mikaboshi's realm to acquire the Grasscutter, a powerful and ancient sword that Mikaboshi's armies of the dead had been trying in vain to acquire. He made an even stronger impression in the Ares (2006) mini-series written by his Marvel co-creator Michael Oeming. Seeing the chaos left behind by the fall of Asgard, Mikaboshi attempted to expand his realm and influence by attacking the struggling Greek pantheon. Mikaboshi's armies continued to hammer away at the dwindling Olympic forces while the crafty shapeshifter attempted to corrupt Ares' son Alexander. Alexander and other Greek gods such as Hercules were able to repel and defeat Mikaboshi at the cost of the almighty Zeus who sacrificed his life in order to ensure their victory.
Mikaboshi would return in the pages of Incredible Hercules where co-writers Greg Pak and Fred Van Lente had the evil shape-shifting god speaking in haikus and elevated his threat level throughout the series to be a danger to the entire Marvel universe. When the Council of Godheads decided to send a team of representatives to find and defeat the Skrull gods during the "Secret Invasion" event, Mikaboshi was chosen to represent the pantheons of the east despite Hercules' strong objections. Although he proved to be useful in the battles the "God Squad" faced during their journey, he faked his demise and once alone, he took command of the conquered pantheon formerly under Skrull control and declared war on any other remaining pantheons within the universe.
While Hercules and his allies averted an apocalypse orchestrated by members of the Greek pantheon, Mikaboshi continued to add pantheon after pantheon to his army of enslaved deities until he set his sight on Earth as the last piece of his reality-conquering puzzle. In the 'Chaos War' event, he conquering the Cosmic Realm as well as the fearsome realm of the frightful dream lord Nightmare, leaving many of Earth's mortal heroes unconscious. As his forces defeated team after team of Earth's remaining heroes, Mikaboshi quickly added the Underworld and its undead armies to his overall army as his goal neared closer to completion with every conquest. At some point, Mikaboshi shed the costume of his Shinto identity and assumed his true form as the fearsome nightmare called the Chaos King, proclaiming his name as he killed the god Zeus a second time.
As the Celestial Axis and most of Earth's pantheons fell one by one, the super-powered Hercules summoned up as much celestial power that he could to fight the Chaos King before he consumed his old enemy Gaea. Hercules and his fellow heroes failed and the Chaos King was successful in absorbing all of reality, leaving behind the same nothingness that had been there before time itself. Unbeknownst to the victorious Mikaboshi, he had been tricked into absorbing an empty dimension that had been created as a sanctuary to evacuate Earth's population in case all attempts to stop Mikaboshi failed. Although the effort cost Hercules his godhood, Mikaboshi was sealed away in that pocket universe and since then, the Marvel universe has had other universe-ending threats to worry about. In the case of Amatsu-Mikaboshi, he may be gone but the Chaos King will never and should never be forgotten.