There are Decepticon legends of an "Ultimate Warrior" and tales of the "Soul of Cybertron". These are merely corrupted myths based on the truth—The Last Autobot!
The Last Autobot is a guardian that Primus prepared, against the day that he would no longer be able to guide his creations. A stoic being, the Last Autobot awaits his destiny.
Marvel Comics continuity
Billions of years ago, before going into his sleep, Primus prepared a Transformer called the Last Autobot. He gave the Last Autobot a measure of his power and set him to wait for the 'chosen one'.
After Primus' death, Cybertron appeared to be tearing itself apart and the Transformers abandoned their home world. The only remaining Transformer was Optimus Prime, whose soul was in the body of Hi-Q.
Prime traveled, along with the Neo-Knights, into the depths of Cybertron where he discovered the Last Autobot. When the Last Autobot awakened, he was surprised to find the form that the chosen one had taken—two souls in one body, a human body. (It's a long story.)
The Last Autobot merged the two souls into one new Optimus Prime body. He revealed that Cybertron wasn't tearing itself apart, but merely reformatting after the death of Primus. The Last Autobot stabilized the process, allowing Cybertron to reform without damaging itself.
After this, the Last Autobot traveled with Prime and the Neo Knights to the planet Klo, where they attacked the Decepticons. The Autobots were being decimated by the Decepticons, and the Last Autobot assisted in the battle—not actively, but by resurrecting all the Autobots who had fallen.
Finally, the Autobots (and presumably the Last Autobot) returned to Cybertron, while the Decepticons exiled themselves to deep space.
- You might find that the Last Autobot's face looks familiar. Can you guess why? That's right, he looks a lot like the Autobot symbol!
- Despite Primus being "neutral" in the war between the Autobots and Decepticons, the Last Autobot is pretty blatantly an Autobot, most likely because he was created eons before the Decepticon movement began and all of Cybertron's inhabitants then were Autobots.
- Vector Prime's design mirrors the Last Autobot in a way, including their similar altmodes and their Autobot symbol-inspired faces. Both also directly serve Primus.
- Simon Furman meant to do more with the quest for the Last Autobot, but the cancellation put paid to that. He stated at OTFCC 2004 that he reused some of his ideas in the War Within stories.