Unicron is a devourer of worlds and the greatest enemy of the Transformers.
Ancient and extremely powerful transforming robot created by the scientist Primacron. Able to transform from planet-consuming planetoid to titanic robot and endowed with vast cosmic power, Unicron rebelled against his creator, leaving him shattered but alive.
Unicron set out on a path of destruction across the universe, fearing only one thing- the Autobot Matrix of Leadership, which somehow had the power to destroy him. In 2005, after consuming the planet Lithone, Unicron made his way towards Cybertron, during which he encountered the broken Decepticon leader Megatron. Offering him a new body and new troops in exchange for destroying the Matrix, Unicron recreated Megatron and his damaged troops as Galvatron, Cyclonus, Scourge, and the Sweeps.
While Galvatron searched, Unicron consumed Cybertron's two moons. Although Galvatron was ultimately successful in tracking down the Matrix, he attempted to hold its power over Unicron, hoping to bend the planet-eater to his will. Instead, Unicron transformed into robot mode and attacked Cybertron, decimating the defending Decepticon (and Autobot) forces. Hot Rod and his fellow Autobots crashlanded within Unicron's body (through one of his eyes), and he forced Galvatron to attack the youthful Autobot. However, the Matrix transformed Hot Rod into Rodimus Prime, who defeated Galvatron and used the Matrix to destroy Unicron from within.
Despite the destruction of his body, Unicron's head and mind survived, trapped in orbit around Cybertron as its new moon. A year later, Unicron was reawakened by a series of intrusions, once by Cyclonus and the Sweeps (in order to determine Galvatron's location from Unicron's memory banks), once by Galvatron and his forces (in order to extract anti-electrons for use against the Autobots), and a final time by Starscream's disembodied spark.
The bodiless Decepticon (who blackmailed Scourge into helping him) offered to make a deal with Unicron for a new body, and Unicron agreed provided he perform three "labors." The first involved the acquisition of the eyes of Metroplex (Starscream got one and replaced the other with one of Trypticon's), so that Unicron could see once more. The second involved bringing Trypticon to Unicron and installing the Decepticon city's transformation cog within his head. The third and final task involved linking Unicron to Cybertron, so that he could transform the planet into his new body. Starscream (who was currently possessing Trypticon) lost control of his stolen body, persuading Unicron to give him a new body early. However, once he did that, Starscream betrayed him, just as energon explosives planted by the Autobots propelled both the head and Starscream into space. Unicron remained in orbit thereafter, but had apparently disappeared by the time of the Maximals and Predacons.
Following his destruction, Unicron's power scattered across the galaxy as the Angolmois Energy, which was collected by the Destron Galvatron in a bid for power. After absorbing the energy and gaining great power, Galvatron was seemingly destroyed, but actually became a thrall of Unicron's mind.
Unicron/Galvatron traveled to Seibertron and attempted (with his Blentron forces) to take the planet as his new body using Vector Sigma, but was ultimately defeated by Lioconvoy, Big Convoy and Magmatron.
Canadian Animated/IDW/Dreamwave comic
Little has been revealed about this Unicron. He began travelling to Cybertron after hearing the unintentional call of Vector Sigma, and appears to have created various advance drones seeking the planet prior to that, such as Scourge.
In Simon Furman's Omega Point series, Unicron's essence was found in a dark cavern by a Predacon warrior. Reshaping it into the Decepticon Matrix of Conquest, the warrior wielded it and became the powerful Shokaract. Shokaract eventually travelled back to the era of the Beast Wars, leading to his destruction and the erasure of Unicron's essence from history. In the new timeline, the future Shokaract found nothing in the cavern.
In Transformers Universe, a far-future Unicron began rebuilding himself by pitting Transformers from various timelines against one another, and feeding on their sparks.
Ancient god of chaos that consumed the universe prior to the present one; his task completed, Unicron slept in the void. However, when he awoke, he found that remnants of the last universe had coalesced into the Big Bang and recreated existence. Unicron immediately began destroying the universe once again, but this time existence had a guardian- the light god Primus.
The two battled on the physical plane to a stalemate, then switched to the mental planes to continue their fight. Finally, Primus was able to trick Unicron, trapping him and his opponent's astral forms within rocky planetoids. However, the chaos-bringer would not be thwarted so easily, and psionically reshaped his prison into a new body, a planet-devouring world that could transform into a titanic robot. Thus Unicron became one of the most feared forces in the universe, consuming every planet in his path.
In 1990, Unicron discovered the location of his ancient enemy Primus when the god was accidentally reawakened by a stray laser blast. Immediately setting course for Cybertron, Unicron created three new servants- Hook, Line and Sinker- and sent them into an alternate-future 2009 to retrieve his future servant Galvatron. Unwillingly, the Decepticon leader agreed to act (along with the other three) as Unicron's herald.
Unicron shut down his mind and drifted the rest of the way to Cybertron, somehow catching the planet's populace by surprise. By that time, a treacherous Galvatron had managed to fully reawaken Primus, who took over the body of Autobot resistance leader Emirate Xaaron. Despite that, Primus' power was so diminished that he was too weak to pose a serious threat, and Unicron destroyed him. Unicron attacked Cybertron, intended to feast on the body of his old enemy, meeting concerted resistance from Autobots, Decepticons, the human Neo-Knights and even the corrupted Creation Matrix. Finally, Optimus Prime was able to purify the Matrix, and use it to destroy Unicron, dying (again) in the process.
Unicron's head drifted through space after his body's destruction, crashing on the planet [Junk] where he dominated the mind of the native Junkions, commanding them to build him a new body. He also took control of Cyclonus, Scourge, and the "freelance peacekeeping agent" Death's Head and sent them to assassinate Shockwave. However, Death's Head was able to enlist the aid of the Autobots, and Rodimus Prime was able to trap Unicron's essence (with Death's Head's help) within the Creation Matrix.
Unicron later managed to possess Rodimus on two occasions and have him attack his fellow Autobots, but he was ultimately able to regain control both times. Despite that, his taint remained in the Matrix, as he was able to indirectly corrupt at least one young Autobot who looked to Rodimus for guidance.