Unicron is the all-powerful, unstoppable Destroyer of Worlds and the brother of the creator-god Primus. Whereas Primus represents the God of the Transformers, Unicron is their Devil. When the first universe was born, The One sent out the astral beings Unicron and Primus to explore it. But wherever Unicron went, he destroyed everything in his path, consuming entire planets. Primus, the Lord of Order, realised that his brother was corrupt and knew that he had to be stopped. In the physical realm, the Chaos-Bringer was stronger than his brother, but Primus shifted their battle on to the astral plane, then back again, only for both their essences to be sealed inside large metallic planetoids. Primus had hoped Unicron would be trapped forever, but over the eons both astral beings psionically molded their respective physical forms into mighty mechanical planets. Primus's body was transformed into the planet Cybertron, whereas Unicron's form appeared as a world-sized, demonic engine of destruction, capable of moving freely through space and eating all planets he encountered. In order to put an end to his brother's rampage, Primus created thirteen robots capable of changing shape to fight Unicron. As the brothers' conflict drew to a close, one of the Thirteen - Megatronus Prime - betrayed his brothers and became an acolyte of Unicron. This traitor would forever more be known as The Fallen. The battle was finally ended when Unicron and the Fallen were banished into a black hole and supposedly erased from reality altogether. Unicron would survive, however, being reborn in another reality, where he would continue to destroy everything before him. But no matter what universe he ventures to, Unicron is always opposed by the children of Primus, the Transformers. They have not always been able to defeat him, though, and Unicron has managed to consume whole universes as a result. As the living embodiment of chaos and destruction, Unicron seeks only to consume world after world across all dimensions, eating until there is absolutely nothing left.
Unicron is what is referred to as a "multiversal singularity", meaning there is only one of him in the entire Transformers multiverse. Though he has appeared in numerous Transformers comics and cartoons, each one is the exact same being but taken on a different form after arriving in a new universe/timeline. If Unicron's body is destroyed, his essence returns to the astral plane and from there he can enter another reality to wreak havoc upon.
Transformers: The Movie
In the 1986 Generation 1 Transformers film, Unicron first appears when he attacks the mechanical world of Lithone. The Lithoneans have no defense against him and try to flee, but only one - Kranix - manages to escape while the rest of his people and his planet are devoured by the Chaos-Bringer. After destroying Lithone, Unicron makes his way toward Cybertron, currently controlled by the Decepticons. On the way, he senses the death of the Autobot leader Optimus Prime, who has passed on the Matrix of Leadership - a portion of the essence of Primus - to Autobot soldier Ultra Magnus. He then came across the bodies of several dying Decepticons that had been jettisoned into space following their retreat from Earth, including Megatron. Unicron greeted Megatron and extended an offer to rebuild and revitalise him and his discarded troops in exchange for their service in hunting down Ultra Magnus and destroying the Matrix. Megatron refused at first, but was tortured into accepting the Chaos-Bringer's terms, and so Unicron rebuilt him into the almighty Galvatron. Unicron then created a starship for his new herald, who ventured to Cybertron to establish his authority over the Decepticon Empire. After Unicron devoured both of Cybertron's moons, Galvatron travelled to Earth to confront the Autobots. He pursued the Autobots across the stars, from Earth, to Quintessa, then Junk, where he finally cornered Ultra Magnus and killed him, taking the Matrix for himself and intending to use it to enslave Unicron. Unicron was outraged by Galvatron's betrayal, but the Decepticon could not open the Matrix and was unable to threaten his master. In response to this betrayal, Unicron transformed into his robot mode and attacked Cybertron, ripping up great chunks of the planet with his hands. The entire Decepticon army scrambled to defend the planet, but Unicron easily swatted them aside like flies and consumed many of them, including Galvatron. The Decepticons who were swallowed would either be dissolved in acid vats in Unicron's stomach or be torn apart by his anti-bodies. Galvatron escaped this fate, however. When the Autobots and Junkions arrived at Cybertron, one of their ships attacked Unicron head-on and crashed through one of the monster's optic lenses. Hot Rod was separated from his comrades and he encountered Galvatron within Unicron's body, and the two Transformers fought for possession of the Matrix while the other Autobots struggled against Unicron's interior defenses. Hot Rod wrested the Matrix from Galvatron and defeated him, then opened the Matrix and unleashed its awesome power which began to destroy Unicron from within. The Autobots managed to escape from Unicron, who in his agony had begun to rip himself apart as the Matrix energies killed him. As Unicron's body exploded, his head was thrown clear and was caught in Cybertron's gravitational field, essentially becoming a new moon and a grim testament to the satellites he had devoured.
- Transformations - Unicron usually has a robot mode and a planet mode. His planet forms across the multiverse have all looked similar with only slight differences between them. In Transformers Prime, Unicron was actually the core of the planet Earth and was capable of forming multiple bodies from rock and earth to fight against Optimus Prime.
- Astral Form - Even if Unicron's body is destroyed, his astral form is eternal and will return to the astral realm upon "death". From there, Unicron can reincarnate himself in another divergent universe to continue his path of destruction.
- Heralds - Like a certain other godlike, world-destroying comic character, Unicron can create heralds to do his bidding. He does this by corrupting Transformers and bending them to his will or by building them within his own megastructure and imbuing them with their own Sparks forged from his own spark core. Unicron maintains a telepathic link with his heralds with near limitless range and can torture them if they get out of line.
- Dark Energon - A corrupt form of Energon that serves as Unicron's lifeblood. Supposedly more powerful than standard Energon and much less stable, it can have radical effects on different machines. In Transformers Prime, it turned the Decepticon ship Nemesis into a sentient being and turned the dead inhabitants of Cybertron into robotic zombies.
- Multiversal Singularity - As a singularity, Unicron has no doppelgangers across the multiverse. Though the inhabitants of various dimensions might interpret his origins in varying ways, there is likely truth to each of them. He can maintain a physical presence in one universe at a time, but his astral essence transcends such barriers.