JANEMPA

Janemba is the main villain of Dragon Ball Z movie 12: Fusion Reborn. He is a demon of pure evil and great power created when a teenage ogre named Saike accidentally destroys all of the evil purifying tanks in hell. The evil forms around him to create a giant, yellow, and childlike being that turns hell into his own paradise. Goku and Pikkon are interuppted from their match in the Other World Tournament to fight Janemba, Pikkon being left behind to free King Yemma from the barrier that Janemba created around his castle. Eventually, Janemba's weaker first form is destroyed by Goku, forcing a transformation to his second state, which enables him to overcome Goku. Vegeta appears and assists Goku, and the two defeat Janemba by fusing into Gogeta, who destroys Janemba by purifying his pure evil energy and turning him back into Saike.

Source : http://dragonballaffanon.wikia.com/wiki/Janemba