Unicron, a living planet who can change into a giant robotic form, is known as the Dark God, the Chaos Bringer, the Planet Eater, and he is dedicated to consuming the multiverse. His goal is to bring an end to the annoying creation boasting independence around him, and find peace by becoming the living center of a swirling, infinite torrent of nothingness at the end of all things.
Frequently, Unicron will make deals with lesser beings, giving them vast new powers in exchange for their servitude. These minions are sometimes stripped entirely of their free will, but others follow him willingly. Service to Unicron, however, is a double edged sword, for it causes insanity and loss of self. And in the end, no matter their intent, Unicron's plans ultimately call for their consumption as well.
G.I. Joe vs. the Transformers
Unicron enslaved the population of the worlds he ate to maintain his inner works; one race included the Battle Beasts. He is known about in Transformer legend, and is said to have been banished from his homeworld.
Thousands of years ago, when humanity was at a primitive stage, Unicron turned up to devour the Earth. The ancestors of Cobra-La, however, had developed a metal-eating spore that could potentially kill Unicron. They came to an agreement—they would go into exile, while Unicron would merely eat human civilisation once it had developed to a useful point. Eventually they did, and Unicron began broadcasting announcements of "I Return" as he headed towards Earth.
Unicron reached Earth and transformed so as to consume civilisation in robot form. Japan was devastated and humanity collectively wet itself. But the world was saved by Cosmos and Flint, who flew into the Chaos-Bringer and doused his organic looking brain with Cobra-La's metal-eating spores. Unicron was killed, the world was saved, and G.I. Joe went on to explore the dead monster's innards.