Primus
Primus is, in almost every single continuity, the creator and very living God of all Transformers.
His specific origins vary from incarnation-to-incarnation, but like Unicron, there are many elements which are similar.
Primus is an incalcuably ancient being (almost always the "God of Creation") who battled the Chaos Bringer, Unicron, at the dawn of time. Despite his vast power, Primus was incapable of defeating Unicron, so he outsmarted his foe, trapping them both into large planetoids.
Over time, both Primus and Unicron learned to shape their prisons from within. Unicron used form to become an immense robotic planet which had the ability to change into an immitation of his former self. Primus, in turn, became the mechanical planet of Cybertron. Knowing that his ancient enemy still lived, Primus created a new life-form from the newly created planet. A mechanical creature capable of changing forms, the very first Transformer. To this Transformer, Primus bestowed the Creation Matrix, a tool which contained the very living essence of Primus.
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.
