Fortress Maximus (Robots in Disguise)
Fortress Maximus was a vastly powerful Autobot city, hidden on Earth long ago. Fortress Maximus' head (a Transformer known as the Emissary) was formed from a combination of the city's central computer with a small Transformer, Cerebros. Cerebros was hidden, with the O-Parts created to guide the Autobots to him via the Orb of Sigma.
Maximus was sought out by a crew that eventually became the Decepticons, and Megatron sought the O-Parts to exploit the city's power. Cerebros was located, and he formed the Emissary and reactivated Maximus. Maximus was theoretically only responsive to human commands, but Scourge had a small fragment of human DNA in him (from the human driver of the truck his stasis pod scanned, Kelly), and was able to control the giant. However, Koji was able to override Scourge's commands (since he was a real human). Fortress Maximus began to wander the world afterwards.
Eventually, Scourge used Carl, a friend of Koji, to control Maximus and have him attack Galvatron, only to have it backfire when Galvatron wiped his programming. Maximus later battled Galvatron again, only to have the Predacon drain his energy. When it was restored by the positive energies of the children of Earth, Maximus gave it to Omega Prime so he could stop Galvatron once and for all.
