Major Story Arcs
Sentinel was downed in combat during the war with Megatron and personally gave Optimus the Matrix of Leadership before he died.
Sentinel was a pacifist, more suited for peace-time leadership than war-time. In the beginning stages of Megatron's new war, Sentinel tried to negotiate with Megatron but was attacked and brutally killed by him. Sentinel wasn't a fool though and didn't bring the Matrix with him for Megatron to just take.
Sentinel became Prime after the disappearance of Nova Prime. He disliked politics and held much disdain for Cybertron's ruling senate. Megatron's illegal activities came to Sentinel's attention and he arrested Megatron and his followers, but the senate forced Sentinel to release Soundwave. The traitor Starscream along with Soundwave successfully sneak attack the senate and release all the prisoners, led by Megatron. Battle erupts and Sentinel dons the Apex armor to fight back. He had the upper hand at first, but ultimately Megatron overpowers him and kills him.
He was originally Sentinel Major, forced to fight in gladitorial games during the time the Quintessons had enslaved the Cybertronians. He helped in the revolt against the Quintessons and forced them off the planet, but their absence caused a civil war amongst the Cybertronians to erupt; splitting the race into the Autobots and Decepticons. During the war, he eventually gained the Matrix and became Sentinel Prime, the sixth leader of the Autobots. The Decepticons had greater numbers and more firepower, so during Sentinel's reign the concept of transforming was devised to defeat the Decepticons through stealth. There was then peace for millions of years, until Megatron rekindled the civil war and defeated Sentinel in combat. Orion Pax would then succeed him as Optimus Prime.
In this version, "Prime" is a lower rank that's held by multiple individuals. Sentinel Prime is an arrogant jerk and often tries to give Optimus Prime a hard time, and tries to impress their leader Ultra Magnus.
Transformers: Foundation (Official prequel to Dark of the Moon)
Sentinel Prime was one of the Primes - a direct descendent of Primus - the first Cybertronian. During a time when Cybertron was low on energon and tribes gathered together to fight over the remaining energon, Sentinel Prime unconvered the Allspark cube from deep within Cybertron. It was believed that the only way to power it was to harvest a star but Cybertron wandered aimlessly through space and the means of interstellar travel was lost with the Primes. One day, Sentinel and his team, which included Optimus, Megatron, Elita-One, Shockwave, and Wheeljack were attacked by a tribe called the Thetacons while they attempt to power the Allspark. Because they could not harvest a star, they hoped that mere proximity to a star would be enough to power the Allspark, so they worked to eventually warp reality and bring a star to Cybertron. As the sun appeared, the battle with the Thetacons ended and they made peace. Ironhide, who was among the Thetacons, asked them what they would do with the Allspark and Sentinel told told them of freedom - a word which had not been spoken since the time of the Primes - and that it was the right of all sentient beings and that it would be their pledge. Later, when the sun set, Sentinel called Optimus to him and told that they would be going to Trypticon. Optimus told Sentinel that he wanted to go to Metrotitan to meet Elita-One’s sisters, but Sentinel replies by telling him that he can do that later and that they need to establish a government as Cybertron had not been united since the time of the Primes. Optimus asks why they have to do this when all the tribal leaders have already given him their allegiance, and Sentinel tells him that he is tired and needs to rest and that it is Optimus’ destiny to lead them and reveals that he is a Prime as well. Hearing this, Optimus didn’t know what to say. Sentinel then asked Optimus what all the war and death they had gone through was, and Optimus replies saying it was evil, but Sentinel tells him it was learning. Despite this, Optimus feels that his destiny is not that of a Prime but to learn more about Cybertron’s past and finding the truth in old stories. Sentinel says that he wished Optimus would decide otherwise and that maybe some day when people speak of Cybertron, they’ll think of science and history rather than war. However, neither of them realized that Megatron was nearby and overheard everything they had just said, filling him with anger because he knew now that Sentinel favored Optimus over him, rather than view them as equals. Three solar cycles later, Trypticon becomes the new capital of the world, and Sentinel sends an invitation to all Cybertronians. He gives a speech, saying that even in the new age of Cybertron, there is still danger and their world is still not tame nor safe. He then declares that Cybertron shall be ruled by two separate bodies working for a common goal; the Autobot science division lead by Optimus, and the Cybertronian Defence Force commanded by Megatron. However, Megatron works to try to give himself more power and later creates an army of followers called “Decepticons” and claims they will stamp out Sentinel Prime. Later, the Decepticons attack the archaeological dig where Sentinel is located at, but Optimus and Chromia arrive and help fight them off. As they retreat to a tunnel, Optimus reveals to Sentinel that they discovered that a marking on his helmet matched with a symbol found in one of the artifacts they had dug up, so Sentinel was right when he said Optimus was a Prime and he accepts it now. Later, as Megatron sends the starship Nemesis into space to find the lost star harvester, Optimus and Sentinel talk about what must be done with Megatron. Optimus feels that Megatron is still a brother to him and that he failed Sentinel by denying his heritage as a Prime and by failing to hold Cybertron together as a result. However, Sentinel says that he knows now and that it won’t be easy but he’ll see what must be done. Later, as Optimus is unsure of what to do, Sentinel reveals that he had finished an invention which was guaranteed to end the war. He says that if Optimus wishes to use it to kill many of the Decepticons, it could do that, and that it offered another path to victory and that Optimus’ forefathers would have been proud of him. However, for his invention to work, its torch had to touch the Allspark in order to activate, so a group of Autobots including Ironhide, Chromia, and Elita-One go to the Simfur temple where the Allspark was located. After getting past Megatron and the Decepticons, Elita-One manages to get the torch to touch the Allspark, but as she heads back to Sentinel Prime, Starscream follows her. As Sentinel receives the torch, he prepares to board to Ark and pilot it to Optimus and orders Wheeljack to take the second craft to Tyger Pax and get the rest of the Autobots. Although Wheeljack initially protests, Sentinel asks him what he’d do to save their world and Wheeljack answers saying he’d do anything, so Sentinel tells him to just do this for him and he promises the war will be over. However, as the Ark approached Optimus, Starscream, who followed the Ark, shot down and destroyed the Ark - seemingly killing Sentinel and the rest of the crew.
Transformers: Dark of the Moon
In the beginning days of the war Sentinel Prime took the Ark and a Special cargo that would save Cybertron but crash landed on the moon in the 1960's causing the "space race" between America and Russia. In present day Optimus, finds one of the Ark's fuel cells in Chernobyl that the humans had kept secret from the Autobots. Optimus and Ratchet go to the moon to retrieve Sentinel and bring him back to earth. Optimus then uses the matrix of leadership to revive Sentinel, who then attacked Optimus upon awakening but the Autobots calm him down and Optimus tells him that he managed to save 5 of the pillars, including the control pillar though Sentinel is annoyed by this as they once had hundreds. Mearing then asks Sentinel what the pillars are and Sentinel explains that they can create a space bridge which could be used to teleport resources or refugees across the universe and demands the humans return the pillars to him and warns them not to let them fall into the Decepticons' hands. Later, Optimus offers Sentinel the Matrix of leadership but Sentinel refuses, saying that he knows nothing of Earth and that he is no longer Optimus' teacher and that Optimus is now his. Later, it is revealed that the Decepticons have the rest of the pillars and that they were waiting for Optimus to revive Sentinel, as he is the only one that can use the pillars. The Autobots then fight with the Dreads and return to base to protect Sentinel. However, at this point Sentinel reveals that he never believed that the Autobots would win the war and that to save Cybertron, he made a deal with Megatron. Sentinel then betrays the Autobots, kills Ironhide, and takes back the pillars from the humans. Later, he meets with Megatron at Washington D.C where he uses the pillars to teleport a bunch of Decepticons that were hidden on the dark side of the moon. Optimus confronts him and asks why he is doing this and Sentinel overpowers him and tells him how they were once gods on Cybertron but on Earth they are nothing more than machines. He spares Optimus' life, believing that he will join him in time. The Decepticons later invade Chicago, and Megatron tells Sentinel how this is the victory he promised him so many years ago. However, Sentinel tells Megatron off and tells him that he has only deigned to work with him and will never work for him. The Autobots later arrive in the city and Sentinel begins activating the pillars with the purpose of teleporting Cybertron into Earth's orbit with the intention of then using Earth's resources and the humans as a labor force to rebuild the planet. However, while Cybertron is being teleported, Optimus shoots down the control pillar, halting its teleportation and then Sentinel begins battle with Optimus. Sentinel ultimately proves superior to Optimus but just as he was about to kill him, Megatron, who had grown tired of taking orders from Sentinel, ambushes Sentinel and brings him down. The Autobots and NEST destroy the control pillar, reversing the teleportation, sending all the Decepticons away, and destroys Cybertron by causing it to implode. Optimus then kills Megatron and Sentinel tries to explain to Optimus that he had to betray him for the sake of their planet's survival, but Optimus tells him he only betrayed himself and then kills Sentinel.