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
The past of Chernabog is shrouded in mystery for now, but soon it will be revealed in due time. All that can be explained at this point is that Chernabog was once the ultimate demon king of the universe, before the Great Old Ones, his servants, betrayed him and locked him away in Bald Mountain. But Chernabog's evil would not be contained for long, and it eventually twists the Horned King and Master Xehanort into becoming the monsters they are now.
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.
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.