Chernabog

Chernabog is the demonic denizen of Bald Mountain, which can be found in the End of the World. Although he is a difficult adversary, there are different tactics to defeating him quite easily. He apparently is neither Heartless, Nobody nor Unbirth, as he is far too dark to be a Nobody, and shows a strategy in battle, while Heartless all act of instinct. It is likely that he is an entity that somehow found himself within the End of the World.

Attacks and Battle
In this battle, Sora and Party can fly around the area, much similiar to Ursula the second time, and the final battle with Ansem.

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. 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 manners of Aero to knock the party far away from him, at which point he will send out beams of light similiar to the ones Xemnas casts from his Nobody Dragon.

A good way to avoid the first one is to quickly fly away, and press the "triangle button" 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 manuevered past.

As Chernabog is fought while in flight, summons are not available to the party, which can be a serious hindrance. However Sora can get around this by landing on his shoulder or the summit of the mountain. Summoning Tinker Bell can be a tide turning advantage.

Background
Chernabog is originally a Disney villain, from Walt Disney's Masterpiece, Fantasia. In which he erupts from the mountain in a similar way to his game-counterpart. He ensues with his devilish antics at night until church bells ringing "Ave Maria" cast him away in the morning.

An arranged version of the original score from the short is used as the battle music in Kingdom Hearts.

Trivia

 * Chernabog is the only Disney villain with no explained reason for fighting Sora or aiding the Heartless in the series. It is possible that he is such a dark entity already, he independently chooses to aid the Heartless in plummeting the universe into darkness by his own choice. This immense darkness could also be the reason he is the only non-Heartless fought in End of the World, because there's no point to turn him into a Heartless. It is also entirely possible that he is unaware of the Heartless and their goal, and simply fights because his dark and evil nature makes him want to wipe out sources of good and light, such as Sora, who just happens to cross his path.