Sentinel Prime

Sentinel Prime is a legendary warrior thought to have been lost long ago. He once led an expedition that took him fourteen galactic convergences to get an echo of something. Sentinel Prime is Optimus Prime's direct predecessor, mentor, and father figure. Sentinel's weapons include a shield, a double-bladed Primax Blade sword that can be folded into one, and a cosmic rust cannon that causes any metal object that it hits to rust into oblivion, as evidenced in the third movie when he shoots Ironhide once in the chest and twice in the back. His intention is to restore Cybertron to its former glory after the war, driven by desperation he turned him against his ideals by siding with the Decepticons. After teleporting hundreds of Decepticons to Earth, he has a confrontation with Optimus, but spares him, in the hope that he would recognize the moral of what he was doing. He demands the exile of the Autobots, who are apparently destroyed while leaving the planet in their space ship. Sentinel betrays Megatron and assumes command of the Decepticons and oversees the final phase of the plan in Chicago.

During the final battle, Sentinel battles Optimus, who faked his death, along with all the Autobots. Sentinel proves to be more than a match for the multitude of Autobots and human soldiers. He appears to have gained the upper hand and is on the verge of executing Optimus when Megatron intervenes and beats Sentinel into submission. The pillars are disabled before Cybertron can be teleported completely, which destroys the wayward planet. Megatron offers Optimus a truce, citing that his only desire is to be in command. Optimus, knowing what Megatron has in mind, refuses and swiftly kills Megatron. Sentinel pleads to Optimus to understand why he had to betray him, but Optimus refuses to condone it and promptly executes Sentinel.

Trivia

 * Sentinel transforms into a Rosenbauer Panther fire truck.
 * Sentinal Prime will meet Thomas, Twilight Sparkle, and their friends in Thomas and Twilight Sparkle's Adventures of Transformers Dark of the Moon.