Unicron is a gargantuan robotic organism.
Unicron is the enemy of all Transformer factions, being the fabled destroyer of Cybertron.
His existence goes back probably to the very creation of the Transformers; however, it is debated as to whether he had disguised himself as a moon to transcend the countless ages or was trapped in that form by Primus. It is possible that he is the largest and most powerful Transfomer in existence (with the exception of Primus who is secretly the planet Cybertron itself and larger although not quite as powerful). Unicron having the ability to resurrect, create vast armies of drones and give immense power to whomever he wishes. One example of this is Unicron finding Megatron's dying body floating through space and in exchange for his services repaired and upgraded Megatron into the far more powerful Galvatron.
He has been defeated many times but never totally destroyed; one of these was when both the Decepticons and the Autobots set aside their war and joined forces to defeat Unicron and stop him from destroying Cybertron.