In Fall Of Cybertron, where does Metroplex actually come from? He first appears just sitting within a stone's throw of the Ark, no one seems to know he exists, and he seems to imply he's a direct servant of the Primes.
Dear Titanically Twisted,
Metroplex was indeed a servant of the Primes, specifically Alpha Trion. Back during the Age of Origins the world was still wild and dangerous, making the prospect of building civilization a difficult one. In this, Alpha Trion recruited Metroplex, a recently forged and gigantic warrior, to act as guardian for his newly-planned settlement. Within Metroplex's citadel form, Alpha Trion would build libraries, schools, and medical facilities, all to help the establishment of a golden age. Many bots flocked to this site, which Alpha Trion named Iacon. The city expanded, new buildings being established outside the citadel as the population grew, new fortifications to drive back the wilderness. In the end the citadel of Iacon became a forgotten relic, and Metroplex assumed a deep slumber.
Then came the great war. With Megatron's many invasions and sieges, Iacon's defences were destroyed. Old facilities were brought back online, including the citadel. When Optimus Prime was planning the exodus, the wide forum outside the citadel was chosen to build the Ark's launch platform, in the hope that the ancient weapons of the citadel could defend it. Indeed they would, as the desperate refuelling of the citadel stirred Metroplex's spark—and with the power of the Matrix, he was roused to his full strength, allowing him to drive back Megatron's penultimate assault on the Ark.
