Information submitted by Nick Peron.
Finding that he could shape his prison, Primus molded Cybertron out of himself. To the first Transformer he gave the Creation Matrix, which contained Primus's essence and was capable of instilling life into machines. As it turned out, it was also the only thing that could kill Unicron. He then went into slumber to avoid psychic detection from Unicron, planning to wake up when he thought his children were ready to destroy his old enemy.
Unicron, meanwhile, shaped his prison into a mechanical planet that could transform into his old form, and began searching for Primus. Millions of years later, Jazz, Bumblebee, Grimlock, and the Rescue patrol were heading to Earth from Cybertron, via the trans-dimensional portal. Bludgeon, Octopunch and Stranglehold then attacked the base and damaged the portal control console, sending both themselves and the Autobots into the center of Cybertron. There, the Autobots found the sleeping Primus and his assigned protector. The guardian then told them the story of Primus's origin and warned them against waking Primus, shortly before the Decepticons caught up and killed him. During the fight, a blast from Octopunch's gun bounced off Grimlock's back and hit Primus in the forehead, waking him up with a scream. Although Primus fell back asleep, and the Transformers were transported to Earth by Xaaron, it became apparent that the scream may have alerted Unicron to their location.
Knowing that the only thing that could stop Unicron was the Creation Matrix, and that the Matrix had been launched into space along with Prime's old body, the Autobots scearched for it. The Decepticon Thunderwing discovered the Autobot operation and decided, with his troops, to pursue them. When Grimlock, Jazz, and Bumblebee finally located the Matrix, Thunderwing defeated them all and took the Matrix, which accepted him. Prime fought Thunderwing for the Matrix, but found himself unable to bear risking the safety of the Matrix. Nightbeat then took matters into his own hands and harpooned Thunderwing with a shuttle harpoon. He then launched the shuttle, along with Thunderwing, into space, where it self-destructed.
Not long after, a Galvatron from an alternate reality was brought into this universe by Unicron. However, he turned on Unicron and instead, convinced Xaaron to wake Primus again. Succeeding, Xaaron was in turn possessed by Primus and transported to Cybertron all the Transformers from Earth (except Fortress Maximus) to fight Unicron. During the battle, Primus attempted to destroy Unicron directly, but was killed (and Xaaron's body destroyed) in the fight. Unicron was eventually annihilated when Prime plunged the Matrix (taken from a returned and destroyed Thunderwing) into Unicron's body.