Primus original
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 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 sonically 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 - rather than transforming his own body, he would create small beings that would be able to change their shape, like Unicron. After performing a "test run" on the moon of Protos, where he successfully created twelve transforming robotic beings, Covenant he birthed from his own body a group of thirteen robots that possessed the ability to change shape, like Unicron. These were the 13 original Transformers, each one infused with a fragment of Primus' life essence known as a spark.

The Thirteen were Primus' soldiers in his war with Unicron, which came to its seeming end during a climactic battle in which one of the Thirteen, who would forever 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 Transformer race grew, and Primus fell into the realm of legend, with a portion of his power, the Matrix of Leadership, handed down through the generations, serving as the Transformers' ever-present link to their creator.

The first time we ever heard the Primus/Unicron backstory was in the UK comic continuity, from Unicron himself, in the story 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 Armies against his mortal foe, Primus, Lord of the Light Gods (note the plural). Events proceeded to play out basically as described above, though the role of the Light Gods and Dark Armies 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. (Only one of the other gods was ever named - the Chronarchitect. How exactly he factors into the current iteration of the myth is unknown.)

The second time the story was told was in the US book by the Keeper, an ancient mechanoid who guarded Primus' 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 US 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 uncreation 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 G1-oriented media, but from a set of Armada trading cards released by Fleer. It was the back story 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 telling 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 Publications' Cybertron comics.

Eventually, millions of years later, in one version of Generation One continuity, Grimlock, Jazz, and Bumblebee found themselves transported to the center of Cybertron, where they discovered the sleeping form of Primus and were told the tale of his and the Transformers' origins by the Keeper. Subsequently, Bludgeon's Pretenders attacked, and during the battle, 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. Primus himself immediately slipped back into his sleep, but it was too late. Primal Scream
Emirate prmus

Emirate Xaaron being possessed by Primus

Unicron dispatched a group of heralds to attack Cybertron and pave the way for his coming, including a Galvatron plucked from an alternate future. Galvatron, however, was not content to serve under Unicron, and instead captured Autobot leader Emirate Xaaron, forcing him to fully awaken Primus, who possessed Xaaron's body and summoned Transformers from all across the universe back to Cybertron. Out of Time! Primus intended for all his children to battle Unicron together, but when the planet-eater arrived, sheer terror prevented them from carrying out his desire. Primus instead made a stand himself, attempting to dupe Unicron into believing he still retained his full power, but Unicron saw through the deception, and destroyed Xaaron's body. Optimus Prime subsequently used the Matrix, the full measure of Primus' power, to destroy Unicron. On the Edge of Extinction! His body in this universe obliterated, Unicron moved on to another dimension, to ravage other worlds, and make further attempts on Cybertrons throughout the multiverse.

(Note: As originally written, it is quite clear that the story intends for Primus to have died after his battle with Unicron. As the current retcon makes it clear that this cannot possibly be, one must assume that Prime was wrong in his analysis that Primus had died. Notably, even in the original comics themselves, the Matrix - the life-force of Primus - is shown to still exist and is functioning fine in the G2 Marvel Comics series that follows his supposed death.)