Primus
Primus is Unicron's sibling and the creator of the Transformers. His body is the planet Cybertron.
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
Universe
When Unicron began gathering Transformers from the multiverse, forced 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.
(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 threaten 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.
