Unicron, also known as The Lord of Chaos serves as the embodiment of destruction while his brother Primus represents the exact opposite of Order. Unicron was first created by an extra-dimensional entity known as The One who visited the Transformers Universe in curiousity. He then divided Unicron into two beings giving creation to Primus. However, it became evident to Primus that in order to assume Godhood within a place called the Omniversal Matrix he had to destroy his brother. A cosmic battle was raged between the two with Unicron gaining the upper hand, badly injuring Primus.
However Unicron was defeated by his brother after they entered the astral plane, Primus imprisoned him and his brother within separate asteroids. Though as time passed Unicron learnt to alter the structure of his prison and craft a metallic body to resemble his humanoid form. Primus also did this and transformed into the Transformers home planet of Cybertron. He placed his life essence with an artefact called The Matrix of Leadership and then created the Transformer race beginning with the Original Thirteen Transformers. After this Primus entered a deep sleep to conceal his presence from his brother.
Unicron began to travel the cosmos consuming planets and satisfying his insatiable hunger, but he soon became aware of his brothers presence during a battle between the autobots and decepticons. In which Grimlock was shot at but the attack deflected of his Titanium back and rebounded onto Primus causing him to scream.
Unicron first appeared in our galaxy at the height of the Cybertronian civil war in the year 2005. The Decepticons had established control over the majority of the transformers' home planet, Cybertron, and the Autobots were forced into various bases hidden around Cybertron, on orbiting moons, and on Earth in the Autobot City, Metroplex.
He made his presence know to the humans and transformers by devouring the Autobot's Cybertronian Moonbase 1. Around this time, the current leader of the Autobots, Optimus Prime, had suffered fatal wounds in battle with the Decepticon general, Megatron and had passed on the Autobot Matrix of Leadership to a new leader, before dying. Seemingly omniscient, Unicron became aware of this transaction and set a course for Earth.
Meanwhile, the Decepticon lieutenant, Starscream (who was next in command after Megatron), had ordered Megatron's disposal by dumping his still conscious body from an airlock on the way back to Cybertron, along with several other badly injured soldiers. The discarded warriors drifted through space towards a silent, waiting Unicron. Upon their arrival (which Unicron claims to have intended), he strikes a bargain with Megatron to seek out and destroy the Autobot Matrix of Leadership, revealing that it is the one and only thing in the universe that can stop him, in exchange for a new body and troops to command. Megatron reluctantly agrees after being briefly, psychically tortured by Unicron. Megatron and his injured officers are given sleek and improved new bodies, along with new monikers and a new ship.
Megatron, now known as Galvatron, sets out to destroy the new Autobot leader, Ultra Magnus, using intel given to him by his new master. Galvatron returns to Unicron after destroying the ship that Ultra Magnus was aboard and is again psychically tortured; this time for his incompetence and eagerness to jump to conclusions. Unicron informs him that Ultra Magnus still lives on a planet of junk, where his ship crash-landed after Galvatron's assault on it. Galvatron is successful the second time. However, tired of his master's cruelty, he instead opts to use the Matrix as a tool to enslave Unicron.
Infuriated by Galvatron's betrayal and arrogance, Unicron transforms into a large, menacing robot and begins to wreak havoc on Cybertron, raking huge fingers across its landscape and shooting defensive airforce units from the sky with eye and mouth beams. An approaching ship full of Autobots, including Hot Rod, is consumed by Unicron.
Eventually, Hot Rod reacquires the Matrix and successfully opens it within Unicron. The unleashed power of the Matrix shorts out Unicron's main processor and he begins to mutilate himself in an attempt to locate and dispose of the body part containing Hot Rod and the Matrix. After the Autobots who were swallowed regroup and escape from his body, Unicron utters his final words, "You cannot destroy my DESTINY!!!", and explodes - The only surviving remnant is his head, which now orbits Cybertron as an eerie reminder to the Transformers of their darkest hour.
Unicron has also appeared in the Transformers Armada Universe. This is however not a different Unicron, There is only one Unicron across the transformers Multiverse.
This time Unicron has been waiting thousands of years slowly feeding off the hatred of the Cybertronian Civil war. During Starscream's dying breath he fired a energy beam using all of his remaining energon. However Unicron simply fired a much larger, much faster beam at Starscream killing him. Then Unicron began to activate prompting the Galvatron And Optimus Prime of this Universe to form an alliance to stop Unicron. However their efforts were futile due to Unicron's nearly impenetrable hide. Then he retaliated crushing many of his attackers then he concentrated his efforts on Cybertron. However Galvatron and Optimus Prime entered
Unicron in an attempt to stop him, but Unicron imprisoned both and gave Optimus the option of merging with him. Naturally Optimus refused, this prompted Unicron to take control of the minicons who turned on Alexis, Rad, and Carlos. But when the three children reasoned with the minicons they turned on their former master and activated the minicons inside unicron who began to drain the life out of the dark God. Then Galvatron engaged Optimus in a fierce duel but the hatred Galvatron was giving Optimus was reviving Unicron. Galvatron committed what appeared to be suicide to destroy Unicron. For now.
In Transformers: Prime, Unicron long ago fought against Primus until he was defeated when Primus created the original 13 Primes who helped defeat Unicron and banished him from Cybertron. Unicron would float endlessly through space and as time passed, he would collect space debris until eventually all the debris collected on him turned into planet Earth. Unicron would lay dormant as the center of Earth until the planetary alignment prophesied by Primus occurred. The changes in gravitation and other forces created by the planetary alignment caused Unicron to awaken and in doing so, natural disasters began occurring all over the Earth's surface. Megatron would try to become his servant but was rejected by Unicron for failing to kill Optimus Prime. Unicron then uses his control over the Earth to create images of himself to fight against Optimus and the Autobots. Eventually, Megatron makes an alliance with the Autobots to stop Unicron and they travel to the Earth's core to shut him down - but not destroy him because he is an essential part of the Earth and without him, Earth would perish. They fight their way through Unicron and eventually reach his spark. Here, Unicron attempts to take control of Megatron through the Dark energon that flows through him but Megatron manages to fight against it and Optimus uses the matrix of leadership to shut Unicron down.
The appearance of Unicron varies between the companies which published Transformers. During his original appearance published by Marvel UK he stood at sizes which dwafted many planets in size. However this has changed in more recent publications.
Unicron is possibly the second most powerful being in the Universe (surpassed only by his father). He is capable of transversing to different realities and consuming them. He is most known for consuming planets which he is able to convert matter into energy. According to the Tranformers handbook, he has cosmic awareness and was said to be able to create life and also dark worlds. It also records him adapting and growing stronger and projecting energy. He possesses enough strength to rip planets out of orbit showing a display of strength beyond 100 tons. During the armada series the mini-cons launched an combine assault which Optimus Prime said could destroy the universe. Unicron was badly damaged however he still survived. In the movie he was able to upgrade and recreate Megatron turning him into Galvatron. In Transformers: Prime, since he is actually the center of the Earth, he has shown that he has complete control over the planet. He caused natural disasters to occur all over the planet and also created images of himself out of stone. These images varied in size as some were about as big as Optimus while one was so big it dwarfed even the Autobots. His blood is also a substance known as dark energon which has the power to raise the dead. When Megatron injected himself with dark energon, he showed that he could also control any dead being who was revived by dark energon.