Unicron is a fictional character from the Transformers universe and toyline. Created by Floro Dery, he was introduced in the 1986 animated film The Transformers: The Movie and has since reappeared in Transformers: Armada, Transformers: Energon, Transformers: Cybertron, Transformers: Prime, Transformers: The Last Knight and Atari's 2004 Transformers video game. Unicron is a prodigiously large robot whose scale reaches planetary proportions, and he is also able to transform into a mechanical planet. Unicron's origin has expanded over the years from simply being a large robot to being a god of chaos who devours realities. He often employs the help of Decepticons in his work, and in some stories is considered part of the origin of the Decepticon forces.
According to the Transformers lore, before the dawn of time, Order and Chaos exist within an extra-dimensional entity known as The One. To explore the fledgling universe, he creates the astral being known as Unicron, and then subdivides him, creating his twin, Primus. Both brothers are multiversal singularities, unique in all realities, but whereas Unicron can only exist in one universe at a time, moving freely between them at will, Primus exists simultaneously in all realities at once.
As Unicron and Primus venture through the cosmos, it becomes apparent to Primus that Unicron is nothing but a purely corrupt being, and so confronts the threat his sibling poses. In combat, Primus is no match for Unicron; in cunning, he proves himself to be his brother's superior when he shifts their battle to the astral plane, and then back to the physical world, sacrificing most of his strength to manifest their essences within metallic planetoids and leaving them both trapped in the planet prison. Primus hopes to contain the evil force forever. Unfortunately, over time, Unicron learns to physically shape his prison into a giant metallic planet, and Primus emulates his brother. The Primus planet becomes the mechanical world of Cybertron. When Unicron learns to transform his planetary form even further, into a gigantic robot form, Primus adapts the idea, and creates The Thirteen, a group of robotic beings that possessed the ability to change shape.
The war between Unicron and Primus comes to its seeming end during a climactic battle in which one of the Thirteen, Megatronus Prime, who is afterwards known as The Fallen, betrays Primus and becomes an acolyte of Unicron. The battle ends when the Fallen and Unicron are sucked into a black hole and disappear. With Unicron gone, Primus enters an eons-long slumber, preventing Unicron from detecting him through the mental link the brothers share.
The Primus/Unicron backstory has evolved and been rewritten a number of times since it originated in the Marvel Comics series. The version recounted above is the current iteration, which has slowly solidified across a wide swath of media (beginning around the time of Transformers: Armada). Before the Primus/Unicron mythos reached its present form, it went through several distinct versions in the Marvel Generation 1 comics and elsewhere.