Sentinel Zeta Prime

Sentinel Zeta Prime, also known as Zeta Prime, or Sentinel Prime was Optimus Prime's predecessor as Autobot leader, and the anti heroic antagonist of War for Cybertron 's Decepticon campaign.

War for Cybertron
When Megatron launched a raid on Trypticon station, and Starscream joined the Decepticons, Jetfire escaped to warn Autobot leader Zeta Prime that the Decepticons had access to Dark Energon. Zeta Prime sent a large group of Autobot soldiers to guard the Energon bridge that Jetfire knew Starscream and his crew would be reactivating in order to refine an endless supply of Dark Energon for the war effort, ordering that all Decepticons would be shot on sight, but they were unsuccessful. After the Decepticons began manufacturing Dark Energon in mass quantities, Megatron led the attack on the Autobot capitol of Iacon, intending to take the Omega Key, a tool that would allow access to Cybertron's core. Megatron's plan was to pump the powerful Dark Energon into the core in the hopes of reviving the planet and restoring it to what he claimed would be its "former glory". He found the Omega Key's resting place...but it was gone, Zeta Prime having anticipated Megatron's plan and taken it under his personal protection himself. 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 before it was too late and Zeta would be forced to execute them.

Megatron's team managed to fight their way to Zeta Prime's chamber in the Iacon Vaults, where he was in control of a mechanism that not only rendered him invincible to all attack, but allowed him to create holographic energon clones of himself and manipulate the ceiling to crush the Decepticons. However, the device had a fatal flaw — 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 laughed, mocking Megatron for heeding the words of his misinformed spies. The device Megatron now held in his hands was not the Omega Key itself, but merely the device that activated the Key. Zeta said that Megatron would only have to wait for the Omega Key to come to him.

Zeta Prime was taken to Megatron's Kaon Prison Complex, but elsewhere on Cybertron, Omega Supreme, guardian of Iacon and, as it turned out, the true Omega Key, was activated with a mission: Destroy Megatron.

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. A young soldier named Optimus was forced to take temporary command of the Autobots and lead the defensive efforts of Iacon. After Starscream was defeated in the Decagon's communications tower, a message, seemingly from Zeta Prime, was played to all Autobot frequencies. Zeta reported his location, and claimed that he had acquired information vital to the Autobot war effort. Ratchet warned that it was an obvious trap, despite the confirmation of his unique ID signature, but Optimus determined that if there was a chance Zeta was still alive, the Autobots must try to rescue him.

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 freed the remaining Autobot prisoners and Air Raid (who had helped orchestrate their first escape), then set off to find their leader. Eventually, Zeta Prime was found, being tortured by Soundwave in his laboratory. After Soundwave was bested by the Autobots, in desperation, the Decepticon drew energy from the injured Zeta's spark itself, mortally wounding him, though Optimus took a final bullet from Soundwave for his erstwhile commander. 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. Zeta Prime, leader of the Autobots, was dead.

Optimus took Zeta's body back to Iacon, presented it before the Autobot High Council, and begged them for guidance in these desperate times. The leader of the Council stated that they existed only to choose Primes, and that it was the Primes who led the Autobots. When Optimus responded that there were no Primes left, the Council leader asked if Optimus would still reject his true calling. The Council said that for too long they have watched the Primes fall to greed and pride, but Optimus was one leader whose wisdom and courage inspired and transformed the lives of those around him. Charging him with the mission to remove the infection of Dark Energon from Cybertron's core, the council bestowed leadership of the Autobots onto Optimus, a burden and responsibility he accepted. He was now Optimus Prime, the last 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.