The Autobot High Council's role was to choose the Primes, the leaders of the Autobots, but observed that many Primes they choose eventually fell to greed and pride. However, they kept a close eye on Optimus, a courageous and wise soldier who inspired those under his command, and who was marked by fate to one day assume the mantle of Prime, although Optimus rejected that he was worthy of being Prime.
When war raged across Cybertron between the Autobots and the Decepticons, the other Primes (except Rodimus; he left Cybertron ages ago and Vector Prime; who have been traveling among time and space) were defeated and one of them was captured by Dark Energon infused Megatron. Despite a daring rescue attempt from the Kaon detention facility by Optimus, Bumblebee, Sideswipe and Air Raid, Zeta succumbed to his injuries and went offline.
Optimus brought Zeta's body to the Council and begged their guidance in these desperate times. Alpha Trion stated that they existed only to choose Primes, and that it was the Primes who led the Autobots. When Optimus responded that there were no Primes remaining, Alpha Trion asked if Optimus would still reject his true calling. He said that for too long the Council have watched the Primes fail in their lofty positions, but Optimus was one leader whose wisdom and courage transformed the lives of those around him. Charging him with the duty to keep the Omega Key from the Decepticon hands, the council bestowed leadership of the Autobots and the Matrix onto Optimus, a burden and responsibility he accepted. Alpha Trion declared that Optimus was now Optimus Prime, the last Prime.