Skirmish

Unicron, the Chaos Bringer

The arch-enemy of his brother Primus, Unicron is dedicated to consuming the multiverse. The only thing that he fears is Primus' essence, contained inside the Matrix of Leadership.

In the beginning

Unicron's original body

At the birth of the Universe, an extradimensional entity known as The One created twin astral beings to reign over the cosmos.[1] One was Primus, the Lord of Light and Order. The other was Unicron, the Lord of Darkness and Chaos. Unicron sought to devour everything that exists and become one with The Void at the end of all things.

Primus was no match for Unicron. But Primus did manage to trap both his and Unicron's essences inside metallic planetoids. Over time, Unicron learned to shape his planetoid into a metal being that was able to transform between planet mode and a robot mode which resembled his original astral form. The two brothers shared a mental link. So long as Primus hid himself in slumber, so did Unicron also sleep. Their war ended, for a time, in stalemate.

War of the balance

Primus' Primal Scream

Grimlock, Jazz, and Bumblebee found themselves transported to the center of Cybertron. In a battle with Bludgeon's Pretenders, a laser blast ricocheted off Grimlock and struck the essence of the sleeping Primus, causing a multiverse-shaking Primal Scream that woke Unicron and alerted him to Primus' location.

The battle between Primus and Unicron began anew. Though Unicron would sometimes be defeated, this is always temporary, and there is a whole multiverse of realities to consume. When Unicron is destroyed in one reality, he will respawn in another, ready to eat again.

Marvel Comics

After the Primal Scream, Unicron headed directly towards Cybertron. He created three Heralds to send ahead of him and prepare the Transformers for his arrival: Hook, Line, and Sinker. He also borrowed a version of Galvatron from another dimension's future in which he was successful in devouring Cybertron.

Unicron cultist with mask

(Interestingly, Unicron does not seem to possess his "brother's" ability to create life, instead being capable merely of modifying existing lifeforms to meet his demands.)

Unicron placed a mental command in some Transformers, generating a cult of Autobots and Decepticons. His worshippers gathered to attack the leader of the two united factions, which had gathered on Cybertron when they sensed Unicron drawing near. Their assassination attempt was defeated with the help of Galvatron, Unicron's wayward herald.

Unicron was so unimaginably large that the sight of him froze both Transformer factions into inaction. Unicron, disappointed that victory was this simple, tore apart the planet's surface. It was only the actions of the kidnapped Galvatron that snapped the Transformers out of their collective terror. Even the united Transformers were unable to hold Unicron off. After being fazed by the Neo-Knight Rapture and temporarily shorted out by Circuit Breaker, it was Optimus Prime who flew towards Unicron and opened the Matrix. The life contained inside caused Unicron to explode.

Unicron showed up a few more times in this universe. He showed up again in 2005 in events similar to his appearance in Transformers: The Movie. In 2007, he used mind control to force Junkions to build him a new body out of the planet of Junk. And in the far, far, future, his essence began to corrupt the Matrix itself, turning Rodimus Prime evil and forcing him into isolation.

Transformers: The Movie

Unicron in planet mode

In his first documented appearance, Unicron was known as a giant Transformer which devoured everything in his path. After the Battle of Autobot City, Megatron and several dead Decepticons were dumped in space. Unicron reformatted Megatron into Galvatron and out of the dead Decepticons, he created Cyclonus, Scourge, and Scourge's Sweeps. He ordered them to destroy the Matrix.

Galvatron returned with the Matrix, but intended to use it to make Unicron his slave. Unicron ate him. After eating two of Cybertron's moons, Unicron attacked Cybertron. Shockwave and the rest of the Decepticons, at the time in control of Cybertron, fought back, but this had little effect. Unicron swallowed many spaceships, one of which contained the young Autobot Hot Rod. Hot Rod and Galvatron battled deep inside Unicron. Ultimately, Hot Rod was able to retake the Matrix, change into Rodimus Prime, defeat Galvatron, and use the Matrix to destroy Unicron. Unicron's body exploded, though its head remained intact and orbited Cybertron.

Original cartoon

Primacron was responsible for Unicron's construction.

Unicron had no explained origin in the movie, but later, in the third season episode "Call of the Primitives", his history in the animated continuity was revealed - Unicron was created near the beginning of time by an ancient genius called Primacron, to destroy the universe for him to remake in his own image. But Unicron decided that he would be the one to rule the universe, not Primacron, and rebelled against his creator, voyaging out into the vast expanse of space.

Unicron's head orbited Cybertron through much of Season 3.

His destruction in the movie was not the last the Transformers saw of Unicron - the ghost of Starscream reactivated his severed head and entered into a bargain with Unicron, performing three labours in exchange for the restoration of his body. Starscream and Scourge gathered for Unicron Metroplex's eyes (breaking one and replacing it with one from Trypticon) and Trypticon's transformation cog, and then connected his head to Cybertron, which would become Unicron's new body. Starscream demanded that Unicron restore his body so that he could complete the connection; but once Unicron had done so, Starscream double-crossed him and refused to finish the job. Unicron's head was subsequently blown off into space by an explosion instigated by the Autobots.

Later, when searching for a new supply of anti-electrons, the Decepticons ventured to Unicron's head, where Cyclonus and Scourge accidentally awoke the slumbering demi-god. At the same time, Grimlock - who had gained super-intelligence through an infusion of anti-electrons - had constructed from pieces of Unicron's head the Technobots, and one of their number, Strafe, severed enough of Unicron's neural connections to shut him back down.

In the series, Unicron's role was taken on by Roger C. Carmel.

Omega Point

Beast Wars Megatron and time-traveler Windrazor fight above the Dark Essence

Following his destruction in 2005, Unicron's severed lifeforce was displaced in time to Earth's distant past, prior to the Beast Wars.
It rested in the pit caused by its own impact, quiescent for many millenia, until found by a Predacon known as The Hunter around the Machine Era. Taking the name Shokaract, he used this Dark Essence to conquer, ruling over an apocalyptic 32nd Century future.
Due to severe time-travel mischief, the events leading up to Shokaracts's timeline were averted when the Dark Essence fled Earth from the Beast Wars, possibly to reappear later in Beast Wars II. (see below)

The Matrix of Conquest served as a container for Unicron’s power

Dark Essence

The Dark Essence possessed only rudimentary intelligence, lacking the full force of Unicron's personality. It appears Unicron's Spark remained with his severed head (which has a back-up spark chamber) in 2005, while his power was displaced in time.
Is it unclear if this displacement was accidental, or represents an attemt by Unicron to preserve his power.

Shokaract contained the Dark Essence within a Matrix of Conquest from which he drew his powers. This is very similar to the relationship between Primus and the Matrix of Leadership, where The Matrix holds the majority of his power, leaving Primus himself weak.
The Matrix of Conquest appears to be unique to Shokaract's timeline branch, and would not exist outside of it.

Reunification

Beast Wars Neo gathered Unicron's scattered life-energy (again hidden on future-Earth, again lacking his mind) with his shattered body.
This may be the Dark Essence's destination after it fled Earth's past.

Beast Wars II and Neo

Unicron's show apearance in Beast Wars Neo

After the events of Transformers: The Movie, the Angolmois that gave him life was extracted and sealed in planet Gaia. However, the Angolmois energy surge from the explosion of Nemesis revived Unicron. He created the Blendtrons to bring him the Angolmois Capsules to complete his resurrection. The Blendtrons brought the lifeless body of Galvatron, which Unicron's essence entered to use as a temporary host. After gathering the Capsules, Unicron possessed Cybertron via Vector Sigma to make it his new body. Ultimately, he was finally defeated by Big Convoy's Matrix Cannon.

Transformers: Universe

Unicron's ruined body in Transformers: Universe.

After being defeated by the Wreckers, Unicron floated in ruin. Using what energies he had left, he called Transformers from various corners of the multiverse inside him to do battle. He feasted upon the sparks of the losers, growing ever more powerful. Primus countered this by calling his own army from the multiverse, led by Optimus Primal. Primal was able to set free the captives inside Unicron and begin a new war between the Minions of Unicron and the Children of Primus.

Unicron Trilogy

Armada

Armada Unicron

Animated Continuity

Armada

Disguised as Cybertron's moon, Unicron created the race of small robots known as the Mini-Cons to increase the ferocity of the Autobot/Decepticon war. When conflict exists, Unicron can feed off the energies released.

When the Mini-Cons found their way to Earth, they were followed by the Autobots and Decepticons. Unicron dispatched an agent, Sideways, created from part of himself, to carry on his work. Sideways escalated the conflict by manipulating control of the three Mini-Cons weapons - the Star Saber, the Skyboom shield and the Requiem Blaster, the three keys that were then used to awaken Unicron.

As Unicron turned on Cybertron, the Mini-Cons broke free of his control, deactivating him - but when Galvatron then began his final battle with Optimus Prime, the hatred and ferocity of the fight awoke 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.

Energon

Unicron sported unusual colors during Energon.

Alas, it was not to be, for there is always hate in the universe. Having survived, Unicron started to consume worlds in the far reaches of space, with the Transformers unaware of his deeds. One such unfortunate world was Planet Q, but the general of that world's armies detonated the planet's core as Unicron consumed it, heavily damaging him and forcing him into dormancy - and within Unicron, the spark of Planet Q's leader, Alpha Q, still existed, and recreated his planet's armies as Terrorcons to gather Energon to regenerate all the worlds Unicron had consumed.

However, Megatron's corpse and spark remained within Unicron, and slowly siphoned off some of the gathered Energon, allowing him to be reborn in his own new body. 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. 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. In the ensuing battle between Unicron and the super-powered Optimus Supreme, Unicron's body was successfully destroyed.

But unbeknownst to anyone, he lived on in two forms: his miniscule Spark, and his consciousness, which lay buried in Megatron's mind. With quiet, subconscious nudgings, Unicron led Megatron to a massive reservior of "Super Energon" beneath Cybertron's surface, which transformed him into Galvatron upon his first exposure.
After a series of failed battles, Galvatron proceeded to expose himself to the Super Energon once again, and this time grew to a gigantic height. Unicron seized control of his body again for a new vessel and set out to rejoin with his Spark, but a Primus-empowered Optimus Supreme engaged him in battle once more, and drew all of Unicron's consciousness out of Galvatron and sealed it within his own Spark.

Enraged, Galvatron prepared to destroy Unicron's Spark, but it merged with him, enlarging him again, leading to another battle with Optimus Supreme, this time bolstered by the combined energy of all his troops' Sparks. Meanwhile, Primus merged with the Super Energon, creating a foundling sun which arrived at the site of the battle.Taking control of his body for a brief moment, Galvatron once again sacrificed himself in order to stop Unicron, plunging himself into the infant sun, which ignited and gave new life to Alpha-Q's worlds.

Cybertron

Primus' plan to trap Unicron in the heart of a new sun failed, and instead resulted in a black hole that threatened the multiverse. His heralds Ramjet and Nemesis Prime travelled to Cybertron to unleash the Dead Matrix upon Primus' vulnerable essence.

Vector Prime was able to defeat Ramjet and send him into the black hole in which Unicron was trapped, while Sentinel Maximus with the help of Omega Prime was able to stop Nemesis Prime from unleashing the power of the Dead Matrix on Primus.

The fate of Unicron is still to be determined.