Chernabog

Chernabog, the Black God, is a powerful demon who only comes out at night. He is supposedly the ruler of all darkness and commands all the other villains, only to lose his control over them when they start having civil wars against each other.

Appearing as one of the most powerful villains in all three major villains tournaments, he first appeared in the animated Disney film Fantasia's "Night on Bald Mountain" sequence. He is seen by most as the darkest and most villianous character ever created by Disney.

Non-Disney villains war
In the First War he had a civil war with the Nightmare King and the Emperor of the Night. He was able to defeat the Nightmare King with the help of the firebird.

Soon after as he caused trouble for the alliance of Rothbart and Rasputin, the two joined forces in order to take him out. However Chernabog was able to defeat the sorcerer's combined efforts. Rasputin revealed the Emporer of the Night, who turned Chernabog back into the peak of Bald Mountain.

Unfortunately, with the return of Mok, Chernabog may be freed once again with a certain item that Mok is looking for...the Armaggeddon Key.

Disney Villains War
He commanded Dr. Facilier and Queen Narissa to try and release him from the Underworld. Dr Faciler used a talisman from the Coachman's lair to possibly free him. During Round 14, Chernabog was finally freed and proceeded to bolster up his forces by bringing forth the Cauldron-Born, the Tyrannosaurus Rex, Scar's Ghost, and Pete, who managed to survive his defeat from the Wind Titan but was now given a Dark mode as the Ghost of Christmas Future. Unleashing his final assault upon the world, he took down the Queen of Hearts, Alameda Slim, Zira, and the Fire Demon, Surt. He was finally taken down by Zeus, of all people.

Marvel Villains Tournament
He commanded Avalanche to merge with him so that he could be free from the Underworld. Their resulting fusion changed Avalanche into a reverse-negative colored version of himself. Before he could unleash his devastating attacks on Doctor Doom during the final battle, the mad doctor vaporized Avalanche, and releasing Chernabog from his soul. The demon god rewarded Doom by throwing him into the cosmos, and then later teaming up with Maleficent to defeat Galactus. After the galactic overlord was killed, Mickey showed up to take down Chernabog once and for all. But all the mouse did was pester the demon with magic attacks. Mickey finally realized that in order to defeat Chernabog, he would have to become Julius by merging with the creature just as Avalanche did. Needless to say, Julius Chernabog Mouse is now stuck in Limbo where a new enemy waits for him...Carnage, the spawn of Venom.

When the second war began, Chernabog was still stuck in the form of Julius, but had managed to get along just fine with Carnage, thereby avoiding any possible conflicts between the two. Their stay at Limbo was interrupted when Negaduck arrived, having been banished there by Hobgoblin's interdimensional portal. Chernabog, still weak and mindlessly stupid from the fusion with Mickey, allowed Carnage to finish off Negaduck, then allied himself with Dormammu when the dimension lord revealed himself to the two villains. Currently, it is unknown whether or not Chernabog will separate himself from Mickey to challenge Avalanche in the Queen's dungeon, much like what Ansem did to Riku in KH: COM.