Unicron is the eternal arch-enemy of his twin brother Primus. He is known as the Dark God, the Destroyer, the Chaos Bringer, the Planet Eater, and he is dedicated to consuming the universe. 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.
To undertake this seemingly overwhelming task, Unicron is able to travel across realities at will, a meandering plague upon existence itself. Integrated into his systems are incomprehensible quantum computers which calculate probabilities forward and backwards in time, in perpetuity, giving his processors an ever changing and evolving map of the multiverse.
The only thing that he fears is Primus' essence, contained inside the Matrix.
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.
Unicron appears in every Transformers Movies and Television Programs, and even Video Games and Toylines, and of course, Comics.
Transformers The Movie
In the year 2005, Unicron made his most famous attack upon Cybertron. While heading towards the planet, he consumed the small world of Lithone, making it perfectly clear how great of a threat he was to the entire universe. As he continued on his way, Unicron's powers allowed him to become aware of the death of Optimus Prime, who handed the Matrix of Leadership to Ultra Magnus just before dying, which angered him tremendously. He then came upon the bodies of several wounded and dying Decepticons, who had been dumped into space following a recent battle on Earth. He extended a proposition to the Decepticons' leader, Megatron, offering to rebuild his body and those of his troops in exchange for their destruction of the Autobot Matrix of Leadership. Tortured into accepting, Megatron was transformed into Galvatron, and his followers became Cyclonus, Scourge, and the Sweeps.
While Galvatron set about reclaiming leadership of the Decepticons, Unicron arrived at Cybertron, and proceeded to consume the planet's two moons. Outraged that Unicron would do this to a planet that he viewed as his, Galvatron found himself the victim of mental torture by his creator, and ordered to Earth to complete the task given him. The ensuing chase finally climaxed in a showdown on the planet of Junk, where Galvatron seized the Matrix and returned to Unicron, only to reveal that he intended to use the talisman to make Unicron his slave. Amused by his pawn's arrogance, Unicron transformed into his gigantic robot mode and ate Galvatron. Unicron then tore into Cybertron, as Shockwave and the rest of the Decepticons on the planet fought back, to little effect. Unicron swallowed many spaceships, and one, which contained the young Autobot Hot Rod and his comrades, crashed through Unicron's eye. Hot Rod and Galvatron battled deep inside Unicron, and ultimately, Hot Rod was able to retake the Matrix from the Decepticon. Transformed by its power into Rodimus Prime, he defeated Galvatron and opened the Matrix inside Unicron, destroying his body from within.
Unicron's head was blown clear during his death throes, and became caught in Cybertron's gravity, turning it into a new moon for the planet — a grisly testament to the satellites he had destroyed.
Transformers Beast Wars
When Starscream appeared on prehistoric Earth and was questioned by the stranded Predacons about how he came to be this way, Starscream "explained" that his body was destroyed by Unicron while serving his master, Galvatron. This was, of course, a misrepresentation, and it was revealed later that it was actually Galvatron himself who killed Starscream. When the Vok spoke with Optimus Primal in Earth's pre-history, they took the form of Unicron's head, claiming that they scanned his mind and chose Unicron as an "authority figure". Megatron would later invoke the name of Unicron by referring to Tarantulas as "Unicron's spawn" in a derogatory manner.
Transformers Unicron Trilogy
In Armada, Unicron's first attack on Cybertron in the present day was an indirect one, over a million years in the making, begun when he opted to disguise his dormant body as Cybertron's moon. From this secret vantage point, he implanted his own cells within the planet, where they germinated and grew into the small race of robots known as the Mini-Cons. Unicron hoped to use the Mini-Cons to increase the ferocity of the Autobot/Decepticon war, and feed off of the energies released in the conflict, but an encounter with a group of time-displaced human children planted the seed of sentience and free will within the Mini-Cons, who ultimately rebelled against their creator and fled Cybertron, crash-landing on the planet Earth.
When the Mini-Cons were reawakened a million years later in 2010, they were detected by the Autobots and Decepticons, who followed their signal to Earth. Unicron also picked up the signal, and dispatched his minion Sideways, an inhabitant of Planet X who would occasionally serve as an avatar for Unicron's spark, to carry on his work. Sideways escalated the conflict by manipulating control of the three Mini-Con weapons, the Star Saber, the Skyboom shield and the Requiem Blaster, but eventually arranged for all three to be returned to Cybertron, where he stole them and used them to reawaken Unicron.
Shedding his moon disguise and transforming to robot mode, Unicron turned on Cybertron, only to be met with resistance in the form of the united armada of Autobots and Decepticons, and his own Mini-Cons, who had linked together and formed a giant duplicate of his body. Now at full power, however, it was child's play for Unicron to exert control over his creations, and bring the Mini-Cons under his thumb once more. Again, it was thanks to the Autobots' human allies—the same children who had been sent back in time and given the Mini-Cons free will in the first place—that the Mini-Cons struggled free of Unicron's control, including the three Mini-Con weapons, who broke free and left Unicron's body deactivated. Believing the threat over, Galvatron then challenged Optimus Prime to their final battle, only for the hatred and ferocity of their fight to awaken Unicron once more. To end the circle of hate that gave Unicron life, Galvatron allowed himself to be consumed by the monster, and in a brilliant flash of light, Unicron apparently ceased to exist.
In Energon, Unicron had survived, and for the next ten years, continued to consume planets in other regions of space. One such planet was the world known as Planet Q, whose inhabitants fought back by detonating the planet's core in Unicron's face. Badly damaged by the explosion, Unicron fell into a state of dormancy, but the life essence of the planet's ruler—the being who would come to be known as Alpha Q—continued to exist within him, and began a not-entirely-sane plan to use Unicron's powers to recreate everything the Chaos-Bringer had ever destroyed. Alpha Q: Identity Recreating the armies of Planet Q as "Terrorcons", Alpha Q had them attack Autobot facilities on Earth and throughout the solar system to gather the energon that would be necessary for his plan to work.
Unbeknownst to Alpha Q, Megatron's corpse and spark remained within Unicron, and he slowly siphoned off some of the gathered Energon, allowing him to be reborn in his own new body. To escape Megatron's fury, Alpha Q jettisoned Unicron's head and escaped in it, while Megatron took over Unicron's body, and continued to gather Energon in order to rebuild Unicron to use as his ultimate weapon. As the climax of this plan neared, a joint attack by Alpha Q, the Autobots and their human allies saw all of Earth's Energon channeled into Unicron's head, which Alpha Q then rammed into Unicron's body. The positively-charged Energon of Earth reacted with the negatively-charged Energon running through Unicron, tearing open a fissure in reality leading to a new area of space where planets Unicron consumed were recreated, and sustained through the Energon radiated from Unicron's head, which had now become a glowing red sun.
Unicron's body, damaged once more, lay in the dark reaches near the fissure, and the Decepticons began to raid Alpha Q's new planets for the Energon necessary to revive Unicron again. A Heroic Battle This time, they succeeded, and Megatron directed Unicron's body to retrieve his head, extinguishing the Energon Sun and killing Alpha Q in the process. But the power of Unicron proved too much for Megatron to control, as Unicron began to take over Megatron's body. The Autobots made a valiant but futile attempt to stop him, but Unicron decimated them. Just as things seemed hopeless, a surge of power came that allowed Optimus-Supreme to grow to Unicron's size and powered up the other Autobots, and after a long and hard battle Optimus destroyed Unicron's body.