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...

History in Kingdom Hearts
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 just large quadrants of anti-matter space that had to be destroyed for colonization, but to the residents of that realm it was genocide and mass destruction. They were formless beings of energy, but this devastating incident was an act of war because all that anti-matter WAS their source of life and energy. So the "Great Old Ones", god-like masters of the Realm of Darkness led by the dark demon Chernabog, concieved of a brilliant plan for retaliation 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 fueled 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, beginning the terrible conflict of the Keyblade War.

Marlwoleath led the charge, as he deployed his forces to cause as much carnage, mass slaughter, 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 that 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.

And all this time, Chernabog has watched his old pawn grow more vengeful at the Realm of Light with each passing millienium. He knows full well what would happen to the Light and Darkness if the Horned King fulfills his revenge plot at last. And he's totally consent with the idea, believing that the destruction of BOTH realms would bring the true revenge to both God and the Great Old Ones for defeating him long ago. After all, only he knew what happened in the finale of the Keyblade War. As it turns out, the χ-Blade, the very blade that was forged to stop Chernabog forever, was shattered just after the Great Old Ones sealed him away, and released seven lights and thirteen darkness's, and when the two sets of light and darkness are brought together, the true χ-Blade would return. Chernabog is now aware of this turn of events, and hopes to use his pawns, Master Xehanort and the Horned King, to bring the weapon into his hands once he is released from Bald Mountain. It seems that the realms of light and darkness are going to watch out for Xehanort's true master returning...

Later on, Chernabog is freed from Bald Mountain by the clash of the 7 lights and 13 darknesses, and comes out of his prison...with the corrupted x-blade in his hand! Master Xehanort was shocked that this was not supposed to happen, he was supposed to become the god of the entire universe and bring "balance" back to the galaxy in his image. But Chernabog cackled at Xehanort, saying that the Horned King, Master Xehanort's master, had used him all this time, and that all of Xehanort's schemes were just to facilitate his final return from Bald Mountain. Therefore, all worlds in both universes would not be merged into one where light and darkness are balanced, in a universe under Xehanort's rule. Instead, they would all be destroyed in a finale of pure darkness where nothing would arise or die. It would be...the end of all existence as we know it! Upon this revelation, Chernabog grabs all 13 Xehanorts, crushes them all in the palm of his hand, and absorbs them completely, transforming himself into some sort of human-like hybrid between Young Xehanort and Chernabog. It is in this form that Chernabog begins his final plans for plunging all worlds into the chaos abyss with the χ-Blade at his disposal.

Boss Strategy
Chernabog has many frightening powers to fight if needed. His main ability is to control fire, like Hades, and uses it in various ways such as breathing fire or shooting fireballs from his hands. He can also cause the volcano he's in to erupt with tremendous force, flap his huge wings to create a gale to blow opponents away, and, when in desperate need, summon six orbs of light to attack the trio.

Although his attacks are few, they are very upscale and dangerous to the health bar. The most powerful attack is when he looks as though he is bent over, apparently hurt, but then rises triumphantly, causing the volcano to erupt violently. This attack gets more powerful later in the battle. Another attack would be when he consistently breathes fire, either from left to right, or right to left. When he begins to lose health, he will use variants of Aero to knock the party far away from him, at which point he will send out beams of light similar to the ones Ansem casts from his World of Chaos form.

A good way to avoid the first one is to quickly fly away, and press to signal to Donald and Goofy to follow Sora, so that they will not perish either. The second can be avoided by simply flying behind his head, and merely waiting for him to stop breathing fire. Also, now would be a good time to attack from the back of his skull. The third cannot be avoided, but the fourth can be difficultly maneuvered past.