Billions of years ago, Primus and Unicron fought an evenly matched battle for supremacy. In order to best his nemesis, Primus created the Thirteen, and together they managed to defeat Unicron, who was in his weakened state and left to drift throughout the galaxy. Primus then merged with the already-existent Cybertron's core and lay dormant for many millions of years. As Cybertron entered and continued a civil war, the battle's began to poison the planet's core in turn infecting Primus. Luckily Orion Pax arrived to help clear the core of its affliction. Primus, sensing Orion's pure spark, bestowed upon him the Matrix of Leadership, containing the essence of the Thirteen original Primes, turning Orion into Optimus Prime.
A Decepticon brute thanked Primus that Megatron was still functional, seconds before he was killed by Omega Supreme. An Autobot soldier later yells "Thank Primus you're here!" at Optimus' arrival before being smashed by a brute. Optimus Prime also thanked Primus for the apparent victory over Trypticon, moments before the giant rose again.