Sentinel Zeta Prime

Sentinel Zeta Prime, also known as Zeta Prime, or Sentinel Prime was Optimus Prime's predecessor as Autobot leader, and the overarching protagonist of War for Cybertron, serving as the main antagonist of the Decepticon campaign and the overarching protagonist of the Autobot campaign. 
He was voiced by Troy Baker, who also voiced Jetfire in the same game. 
==History==
===War for Cybertron===
After Megatron raided Trypticon station and Starscream joined the Decepticons, Zeta Prime was warned by Jetfire that the Decepticons had access to Dark Energon. Zeta sent a group of Autobots to guard the Energon bridge that Starscream reactivate in order to refine an endless supply of Dark Energon, ordering that all Decepticons would be shot on sight, but they were unsuccessful. 
Anticipating Megatron's plan to access the Core to Cybertron in order to infuse Dark Energon to it, Zeta Prime took what Megatron was the Omega Key, and put it under personal protection. Megatron simply declared that he would take it from Zeta, who appeared several more times via holographs during the missions, warning the Decepticons to turn back. When Megatron was unable to breach the Iacon Vaults, Zeta mocked him.
After Megatron's team managed to fight their way to Zeta Prime's chamber, Zeta used a mechanism that not rendered him invincible to all attack, and allowed him to create holographic enrgon clones of himself and crush the Decepticons with the roof. However, the device was flawed, as it generated tremendous heat, requiring the core to periodically open up in order to cool down, allowing Megatron and his Decepticons to damage it. As Zeta Prime's machine crumbled around him, he was pinned to the floor by the wreckage. He told Megatron to finish it, but instead, the Decepticon leader squeezed his spark casing, forcing Prime to release his grip on the Omega Key. As Megatron gloated in his victory, the "key" started to flash and beep. Zeta Prime revealed that Megatron's spies were wrong. The device was actually what summoned the real Key, Omega Supreme.
After the defeat of Omega Supreme and the corruption of Cybertron's core with Dark Energon, rumours spread through the Autobot camp that Zeta Prime was dead. Zeta was actually imprisoned in Kaon and was tortured by Soundwave in his laboratory. After Starscream was defeated in the Decagon's communications tower by Optimus, Ratchet, and Bumblebee, a message, seemingly from Zeta Prime, was played to all Autobot frequencies. Zeta reported his location, claiming he had acquired information vital to the war effort. Ratchet warned that it was an obvious trap, despite the confirmation of his ID signature, but Optimus determined to rescue Zeta.
Allowing themselves to be captured, Optimus, Bumblebee and Sideswipe were imprisoned (and then broke free), only to learn from Megatron himself that he had orchestrated Zeta Prime's message, their capture and their subsequent escape, so that Megatron could talk to Optimus in a situation he controlled. After being captured and escaping again, Optimus' team eventually found Zeta Prime, and fought Soundwave. After he was bested by the Autobots the desperate Soundwave drew energy from the injured Zeta's spark itself, mortally wounding him, although Optimus took a shot intended for Zeta. Optimus promised to bring Zeta home, but with his last breath, the Prime said that while he appreciated the noble sentiment, it was already too late, and died in his arms. Optimus took Zeta's body to the Autobot High Council, and the Council, seeing Optimus's wisdom and courage, made him the next Prime.
====DS====
After the Autobots severed the link between the Energon Bridge and Cybertron's core, Zeta Prime ordered Optimus to protect the library while he himself took the Omega Key to a secure location. Little did they know that they were being spied on by Laserbeak, who reported back to the Decepticons. The Decepticons eventually tracked him down to Iacon library where, protected behind a force field, Zeta Prime unleashed defense systems on them. Unfortunately he made the mistake of putting the power sources for the force field outside said force field, and the Decepticons were able to disable it and blast him. Defiant to the last, Prime told Megatron to go scrap himself. To Megatron's surprise, the "Omega Key" turned out to activate the real key — Omega Supreme.
Following Omega Supreme's defeat, Zeta Prime was taken to Kaon Prison. While held there, he attempted to send a distress call to his old friend Optimus. Once the Autobots retook Iacon and got the Data-net back online, he was able to get through and tell them where he was. Optimus and a group of Autobots fought their way into Kaon prison and eventually found their leader on the verge of death. His dying wish was for them to take his body back to the Autobot council.
=== Exodus ===
Note: Exodus tells it's own account of Sentinel's fate, which conflicts with the Cybertron games. 
Though not one of the Thirteen Primes, Sentinel Prime ruled Cybertron since near that time. Having already overthrown the Quintessona during the Age of Wrath, Sentinel oversaw Cybertron's space bridge expansion to worlds such as Gigantion, Velocitron and the Hub, and the construction of Trypticon Station. But as the space bridges grew less reliable and the Golden Age drew to a close, Sentinel reorganized Cybertron into Guild-run castes, an action Alpha Trion opposed, forcing him to retreat forever into the Hall of Records in Iacon. Sentinel also became reclusive, enforcing the notion he did not care for the lower castes.
When the Kaonian gladiator boss Megatron began preaching against the caste system, Sentinel dispatched the security agents Bumper and Fastback to investigate him. When their identities were exposed, Megatron brutally murdered them—an act for which Sentinel blamed both Megatron and himself. It was while attending a performance by comedian Armorhide at Altihex Casino when Megatron's Decepticons attacked. Sentinel did not try to fight back, and relied on his bodyguards Starscream, Thundercracker and Skywarp for his safety—only to discover they were part of the plot, and they delivered him to Kaon to gain favour with Megatron.
Megatron occasionally desired to take out his anger on Sentinel, but Starscream saw him as a potential bargaining chip—perhaps as a trade in a ransom deal—so they compromised and had the Prime moved to Moon Base One. As the war with the Autobots lulled, Starscream visited Sentinel and offered him a chance to redeem his cowardice at Altihex. As Megatron plotted to take the Plasma energy Chamber beneath the High Council Tower, Starscream released Sentinel and he confronted Megatron below Iacon. Though he was an agile opponent, Megatron literally disarmed Sentinel and cut open his chest, exposing the Code Key of Justice needed to access the Chamber within him. Starscream removed the Key and Blackout was ordered to return Sentinel to Kaon.
Sentinel was chained to a pillar in the Underworld pits that he had despised his whole life. But he was able to send a distress signal which Prowl received, and he, Jazz and Sentinel's replacement Optimus Prime arrived to rescue him. Sentinel said he had overstayed his welcome as Prime and said the High Council made the right decision in appointing Optimus. Before dying, he ordered Optimus to journey to Cybertron's core and purge it of Megatron's Dark Energon. Optimus returned Sentinel's body to Iacon, where it was laid to rest.
==Gallery==
zeta prime alt.png|vehicle mode
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