Janemba is a powerful reality bending demon who appeared in the 12th Dragon Ball Z movie, Fusion Reborn.
In "The Fusion of Two Warriors", he was created when Olympius unleashed a thousand years of evil unto a young ogre, turning him into a large obese, yet childish monster. Janemba then wreaked havoc upon both the mortal realm and the underworld with his reality warping abilities. The Guardians, teaming up with the Ultimate Defenders managed to beat Janemba with their combined efforts.
But Janemba ended up transforming into his super form, making him more fierce, more powerful and more dangerous. It took the fusion between the Dragon Halfa, Terrence and the Half Saiyan Ultimate Lifeform, Ethan, to defeat the demon.