“I too was once THE Prime… now I am but a Sentinel, guarding a world long abandoned…” – Sentinel Prime
In the time when humanity was unknown, when the Milky Way was unknown to many, there was yet a successor to Prima. A singular soldier, schooled in both the ways of peace and of war, a veteran of the battles that forged a single unified Cybertron: Sentinel. Before either Autobots or Decepticons were created, he led Cybertron’s people in a wave of expansion and creation that led to the Golden Age of Cybertron.
Then came the Great War. Autobots and Decepticons, fighting each other and other races, destroying all that he had built. Sentinel Prime, hating the race he had brought to this pass, left Cybertron, vowing never to return. He left in his stead the new Optimus Prime, empowered by the legacy of the Matrix of Leadership. Believed dead by all of Cybertron, Sentinel fled to the farthest reaches of the galaxy, thinking himself a failure. At some point, unknown to even Vector Prime, though, he changed his mind. Returning to civilization, he landed on Earth during the early 21st century, where he and Optimus brought an end to the Great War.
Afterwards, he surrendered his title of Prime, intending to set up safeguards against such wars in the future. This led him to find that there were many superweapons built by others within range not only of Cybertron, but of humanity as well. He lead a force to destroy some of the oldest structures, long abandoned by their creators, however, he was attacked by an alien fleet upon reaching the facility. Sentinel disabled the fleet, but was driven off. He later obtained his Cosmic Rust Cannon from a similar facility, which he managed to destroy entirely.
Upon the outbreak of a war between the same aliens and the human military, Sentinel and a combined force of Autobots and Decepticons joined the humans, fighting an ongoing defensive war to protect civilian installations. At the time, Optimus restored Sentinel’s Primehood, though his teacher initially refused the offer.