Darkest Hour

S2, E26: Megatron activates the omega lock and starts restoring Cybertron, but he then reveals he is also going to use the lock's power to turn Earth into a mechanical planet, which would kill all of its inhabitants. Optimus Prime destroys the lock before it can finish its restoration, saving Earth but dooming Cybertron. The Autobots retreat back to Earth, only to find a gigantic fortress has appeared outside their base, and the Decepticons have finally discovered the base's location.