Primus
Primus is Unicron's sibling and the creator of the Transformers. His body is the planet Cybertron. Primus' lifeforce (aka the Allspark) is a multiversal singularity; it exists across all Transformers realities.
In the beginning
At the birth of the Universe, an extradimensional entity known as The One created twin astral beings to reign over the cosmos. One was Primus, the Lord of Light and Order. The other was Unicron, the Lord of Darkness and Chaos. Unicron sought to devour everything that exists and become one with The Void at the end of all things.
Primus was no match for Unicron. But Primus did manage to trap both his and Unicron's essences inside metallic planetoids. Over time, they learned to shape their prisons. Primus became the mechanical planet of Cybertron. Knowing that his ancient enemy still lived, Primus created 13 lifeforms from the material of the newly created planet. Mechanical creatures capable of changing forms, the first Transformers. To the very first of these 13, Primus bestowed the Matrix, his essence. These Transformers would, in time, grow strong in number and knowledge and power and be able to defeat Unicron.
The two brothers shared a mental link. So long as Primus hid himself in slumber, so did Unicron also sleep. Their war ended, for a time, in stalemate.
War of the balance
Grimlock, Jazz, and Bumblebee found themselves transported to the center of Cybertron. In a battle with Bludgeon's Pretenders, 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.
The battle between Primus and Unicron began anew. Though Unicron would sometimes be defeated, this is always temporary, and there is a whole multiverse of realities to consume. When Unicron is destroyed in one reality, he will respawn in another, ready to eat again. Primus will always be waiting, hoping to incapacitate Unicron and preserve existence.
Marvel Comics Continuity
Beast Wars/Beast Machines
Primus' name was often invoked in prayer or in righteous anger by the Beast Warriors.
The Covenant of Primus, a prophetical set of data tracks, were on board both the Ark and the Nemesis when they crashed 4 million years ago on prehistoric Earth. Megatron himself took his name from these tracks (as did, presumably, the original Megatron), and the scriptures foretold the events which completed the Beast Wars.
After the Beast Wars, Megatron intended to incorporate Primus' essence, the Allspark, into his own being and become godlike by stealing every Spark of every Transformer on Cybertron. Ultimately, this was merely one step of the Oracle's plan to reformat Cybertron into a technorganic paradise. After Megatron and Optimus Primal were plunged into the planet's core, the planet was reborn and the abducted Sparks were returned to new technorganic bodies.
Universe
When Unicron began gathering Transformers from the multiverse, forcing them to fight, and harvesting the Sparks of the losers, this presented a huge threat to Primus and the Allspark. Transformers' Sparks are pieces of Primus' essence, so each devoured soul weakened him.
Primus and Alpha Trion called back from the Allspark the now-legendary Maximal ship commander and Cybertron's savior, Optimus Primal, to rescue the possessed, battling captives from the Pit of Unicron -- and to lead a team of multiversal warriors to put a stop to his soul-stealing scheme before it could cause the Allspark to collapse.
Optimus Primal was allowed by Primus to choose for his army any Transformer who ever existed, living or dead. This requires considerable amounts Primus' power, so Primal was told to choose sparingly. Primal chose his departed former troops Rhinox and Depth Charge.
(This storyline was never completed.)
Unicron Trilogy
Energon
Cybertron
Primus' plot to bury Unicron's essence in the center of a newborn star failed. The star collapsed into a black hole which threatened not only Cybertron and the universe, but the multiverse as well. Only four Cyber Planet Keys, hidden on planets throughout the galaxy, combined with the Omega Lock, could awaken Primus and allow him to destroy the black hole.
The black hole posed unique problems. Primus and Unicron -- representing Order and Chaos, respectively -- together forged a delicate balance in the multiverse. With one or the other missing, the balance would be upset, spiralling all that exists into an unknown doom.
In addition, the black hole was positioned very close to Cybertron itself, making Primus, despite Unicron's apparent demise, the most vulnerable he had ever been. Cybertron was the linchpin, the astral center, of the entire multiverse universe, itself another singularity. Unicron's heralds Ramjet and Nemesis Prime arrived on Cybertron to unleash the Dead Matrix on Primus' essence. For the first time ever, Unicron, who is normally limited to threatening Primus one reality at a time, would be able to weaken Primus across all timelines -- and the black hole's proximity would end their cosmic feud forever.
Toys
To date, only two toys of Primus have been released throughout the world. The first was a "Lucky Draw" toy in Japan, a recolored metallic Rodimus Prime that was given away as a contest prize from a collector book.
The second was released in Galaxy Force as "Primus" in Spring 2006, only in Japan. The Western version will be released as "Cybertron Primus" in Cybertron from Hasbro, and the first run will include a bonus accessory of Unicron's decaying severed head.
