Unicron, also known as "The Chaos Bringer" or "The Destroyer" is one of two supreme forces of the Transformers Multiverse. He is the embodiment of chaos and darkness and his only desire is to bring an end to all that exists, and find peace by becoming the living center of a swirling, infinite torrent of nothingness at the end of all things. He is the eternal arch-enemy and brother of Primus, the embodiment of order and light, and thus Unicron is the devil of the Transformers Multiverse. However he was originally created by The One first to act as the supreme overseer of the multiverse but his chaotic and evil nature forced him to create Primus to stop him and supersede him.
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.
Being that he is an entity of destruction, he is incapable of creating life, however in the Tranformers: Prime universe, in which after his defeat at the hands of Primus and his Thirteen, his unconscious body drifted through space and ended up orbiting the sun and his body became covered in rocky debris which then became the Earth, a planet suitable for organic life to grow on which led to its habitation by humans, and this was the only instance of Unicron ever supporting life, albeit it was without his knowing or direct influence. However, while humans are considered to be in a sense Unicron's progeny by Optimus Prime, he sees them more akin to parasites which would be accurate as he was not their creator, simply beings that lived off the resources that gathered around his body. Also of note that this is the only instance in the multiverse in which Unicron has a connection with Earth at all.
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 multiverse singularities, unique in all realities, but whereas Unicron could only exist in one dimension 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—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.