Chernabog

The Demon Lord of all darkness, the Chernabogs that Sora and Riku fought before in the End of the World and the Symphony of Sorcery was merely just shadowy constructs created in his image by Xehanort. This creature is the real deal, and he's not happy for getting sealed away on Bald Mountain for 10,000 years. But perhaps that's because of what's happened long ago in the past...

Over ten thousand years ago, humans from the realm of light began to expand across the boundaries between their realm, and the realm of darkness. To them it was destroyed large quadrants of anti-matter space for colonization, to the residents of that realm it was genocide and mass destruction. They were formless and beings of energy, but this devastating incident was an act of war. So the "Great Old Ones", god-like masters of the Realm of Darkness and led by the darkest one of all with a physical body, Chernabog, concieved of a brilliant plan for retliation against the Realm of Light.

They gave selected beings from their realm physical form, and sent them into the other realm to confront and stop the ones responsible for the destruction, by any means necessary. The first being to be born from darkness was the creature soon to be known as Marlwoleath the Horned King.

Soon more of his kind arrived, and together they became the "Ethereal Blade wielders". They harnessed the power of darkness to create blades of dark energy. With these and other weapons, they initiated a series of attacks on the realm, which at that time only included a single yet advanced world. As the brutality of their attacks escalated, the denizens fought back and harnessed the power of light, and fuled them into strange new weapons called "keyblades". The counterattacks began, and in no short time it became a full blown war between the Ethereal and Keyblade wielders.

Marlwoleath led the charge, as he deployed his forces to cause as much carnage, mass sluaghter, and annihilation as possible on this world and its residents. But they would not yield, for with the power of light they continued on. But the Horned King refused to give it, so he personally began to bring more beings over from the Realm of Darkness. But these beings came out different, he gave them physical form through other means besides the Great Old Ones, to make the influx of Ethereal bladers faster. In the process, they lost contact to the Great Old Ones, with each new squadron they became more deranged, beastial, and less intelligent.

He had created the first "Heartless", beings of darkness so out of touch with their masters, that they devolved into beasts that only craved the hearts of the denizens of Light, taking on the forms of vicious monsters.Marloweath's actions were seen as abominable even by both realms, but he continued on and tried to control these creatures to turn the tide of the war. Eventually he succeeded after several decades, but disillusioned them or had most of his fellow Ethereal bladers killed. As the Horned King tried to make the final push and destroy the Realm of Light, the Great Old Ones intervened, much to their leader's chagrin.

They called for an armistace, and ordered the Ethereal bladers to return. They also used their power to bring the Heartless back to their realm by force, in the hope of reconnecting them and transforming them back from beasts. Marwoleath refused and demanded to finish the war once and for all. But the Great Old Ones still refused. Chernabog, by this point, was furious as to why his own servants would betray their own cause. They said retaliation for the other realm's crimes had been repaid, thus restoring the balance between their realms. To destroy one realm would destroy all of reality. And to prevent another war like this from happening, they would have to betray their master and seal him away in a massive prison for all eternity. Chernabog screamed in fury as he was thrown into Bald Mountain, resulting with the Ethereal Blade wielders and Heartless, as well as their counterparts, the Heart Eaters, all being sealed back into the Realm of Darkness. Once the deed was done, the Great Old Ones felt satisfied and content with what they had done, hoping for the best to come from this.

But the Horned King felt differently, and defied his masters. He escaped their sight and disconnected from the Darkness. Slowly he began to de-evolve, but at a much slower pace than the Heartless. Marwoleath began to prepare for the time when the realm would be in a false sense of safety once again. At this time, his vengeance would be quenched with the total destruction of this world, and the Realm of Light itself. But first...he needs to find a certain artifact which he believes can help him.