Unicron is the eternal arch nemesis of his twin brother, Primus. He is the overarching boss of Megatron and his Decepticons. And he is also the Chaos Bringer, the Lord of Chaos, and the Planet Eater.
Before the dawn of time, Order and Chaos existed within an extra-dimensional entity known as The One. To explore the fledgling universe, he created the astral being known as Unicron, and then subdivided him, creating his twin, Primus. Both brothers were multiversal singularities, unique in all realities, but whereas Unicron could only exist in one universe at a time, moving between them at will, Primus existed simultaneously in all realities at once. It is suggested, in fact, that the two brothers embody the basic concepts of reality—good and evil, order and chaos, light and darkness, life and death, creation and destruction—and that their continued existence is necessary for the stability of the multiverse.
As Unicron and Primus went about their appointed task, venturing through the cosmos, it became apparent to Primus that Unicron was a corrupt being, and he took it upon himself to stop the threat posed to all of existence by his sibling. In combat, Primus was no match for Unicron. In cunning, however, he proved himself to be his brother's superior when he shifted their battle to the astral plane, and then back to the physical world once more, only to have both their essences manifest within metallic planetoids, leaving them both trapped. It was with this act of sacrifice that Primus hoped to contain Unicron's evil forever. Unfortunately for him, over time, Unicron learned to psionically shape his prison into a giant metallic planet, and Primus followed suit, becoming the mechanical world of Cybertron. When Unicron then learned to transform his planetary form even further, into a gigantic robot form, Primus adapted the idea to suit his own ends, creating a group of thirteen robots that possessed the ability to change shape, like Unicron.
The war between Unicron and Primus came to its seeming end during a climactic battle in which one of the Thirteen, who would forever afterwards be known as The Fallen, betrayed Primus and became an acolyte of Unicron. The battle ended when the Fallen and Unicron were sucked into a black hole and disappeared from reality. With Unicron gone for now, Primus entered an eons-long slumber, his self-imposed sleep preventing Unicron from detecting him through the mental link the brothers shared.
The Primus/Unicron backstory has evolved and been rewritten a good number of times since it first originated in the Marvel Comics series. The version recounted above is the current iteration, which has slowly solidified across a wide swath of media. Before the Primus/Unicron mythos reached its present form it went through several distinct versions in the Marvel Generation 1 comics and elsewhere.
The first time the Primus/Unicron backstory appeared was in the UK comic continuity, from Unicron himself, in the story "The Legacy of Unicron!", when he recounted it to Death's Head. Per Unicron's telling of events, he was a primal force of evil at the dawn of the universe, who led a legion of Dark Gods against his mortal foe, Primus, Lord of the Light Gods. Events proceeded to play out basically as described above, though the role of the Light and Dark Gods would diminish with each subsequent retelling of the story, until the current version, in which Primus and Unicron are alone, and have a unique origin.
The second time the story was told was in the United States book by the Keeper, an ancient mechanoid who guarded Primus's head at the center of Cybertron. This telling is effectively the same as the previous UK story, but mentions that their battle was towards the end of the era of gods, that Primus and Unicron were the last of their respective pantheons, and Primus had to defeat Unicron before he could take his place with the other gods in the "Omniversal Matrix".
The third time the story was told was also in the United States Marvel series, this time by Primus, when he gathered all his children together to prepare for Unicron's coming. It was with this telling that we learned that Unicron predated the current universe, and had destroyed the previous universe which existed before the current one. He had slept peacefully, alone in the void of un-creation that remained, until fragments of the old universe that he had overlooked reacted, causing the Big Bang and birthing the current universe. The "sentient core" of this new universe recognized the threat that Unicron posed, and so created Primus to counter his evil and be guardian of the new creation.
The first modern retelling of the origins of Primus and Unicron did not come from Generation 1-oriented media, but from a set of Armada trading cards released by Fleer. It was the backstory printed on Unicron's card which introduced the concept of the two being brothers created to explore the new universe by an extra-dimensional entity, here named the "Allspark".
This was subsequently expanded on and combined with aspects of the various Marvel Comics stories in Transformers: The Ultimate Guide', published by Dorling Kindersley and written by Simon Furman, who had written all three prior tellings in the first place. Here, the entity Fleer had called the "Allspark" was redubbed "The One", and the modern iteration of the myth detailed above was firmly established, and went on to form the backbone of subsequent fiction such as Universe and Fun Publication Cybertron comics.
And he is known to be the source of Dark Energon which can revive the dead as a zombie Terrorcon which could be where the Decepticon sub faction the Terrorcons got their name from.
And he is reprised by Mark Acheson as a tribute to Orson Welles in the English dub and Katsumi Cho in the Japanese dub.