Legend says that Primus created the Thirteen to defeat Unicron. The Thirteen succeeded, and Unicron's defeated form was cast by them into space. Statues of the Thirten were created at some point after they defeafed Unicron What is known about the Primes is that their war made "enemies of brothers, murderers of lovers, heroes of the insignificant, and cowards of the mighty". Allegiances formed and broke apart: Megatronus refused to bow before his brothers, and he believed he would be vindicated someday despite his destructive methods. The murder of one sent them across the stars, to battle among themselves for billions of cycles. Finally, there came a time when Prima had to relinquish the Matrix. Afterwards, the Covenant of Primusdecreed Cybertron could only have one Prime at a time.
Three seconds of one episode made up for a lifetime of lack of Thirteen images.
Alpha Trion remained on Cybertron, recording these events into the Covenant, and the Primes became myths doubted by some. Alpha Trion retreated into becoming the Archivist of the Hall of Records when Sentinel Prime, the High Council and the Guilds of Cybertron ignored his protests about the caste system. The Matrix did not bestow itself onto another until the unworthy Sentinel died, and Optimus Prime was imbued with it, while his nemesis had become known as Megatron, who like his namesake had come to destroy that he believed in.