Complex sentence: In Cybertron's past, he and his partner, Nightstalker, were the bodyguards of the Overlords, the Autobots who had ruled Cybertron for generations, and guarded them until only one, last, enfeebled Overlord remained.
Split into simpler sentences: In Cybertron's past, he and his partner, Nightstalker, were the bodyguards of the Overlords, the Autobots who had ruled Cybertron for generations. The two guarded them until only one, last, enfeebled Overlord remained.