Sentinel Prime

Canon bio
In the Transformers film series, 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 cannon that cause any metal object that it hits to rust into oblivion, as evidenced in the third movie when he shoots Ironhide 5-7 times in the chest. He also carries a double-bladed sword that can be folded into one. His intention to restore Cybertron to its former glory after the war drove him mad and going against his ideals by siding with the Decepticons. Sentinel Prime is voiced by Leonard Nimoy, who previously did the voice of Galvatron in The Transformers: The Movie. In the second film of the series, Transformers: Revenge of the Fallen, Sentinel Prime is briefly mentioned in the rambling speech given by Sam Witwicky while under an All Spark fragment-induced trance, with Sam mentioning a "mission" Sentinel led. Sentinel Prime appears in the third film, Transformers: Dark of the Moon, as the main antagonist, revealed as a rogue Autobot later in the film. Though it was believed that the Space Bridge technology he devised was to turn the tide of the war for the Autobot's benefit, Sentinel Prime made a deal with Megatron to rebuild Cybertron by finding an inhabitable world and to enslave its occupants. However, the Ark, Sentinel's ship, was hit during the war between the Autobots and Decepticons and it drifted through space until it eventually crashed on Earth's moon in 1961. Before the impact, Sentinel hid himself in the crash vault with five of his Space Bridge pillars, including the control pillar, however he soon ran out of Energon and became incapacitated. His body was then recovered by Optimus Prime and Ratchet when the two Autobots come investigating the crash landed ship and was brought to Earth, where he was resurrected with the power of the Matrix of Leadership. Sentinel was distressed that hundreds of his pillars were missing, and warned Charlotte Mearing that the Decepticons could destroy Earth with the pillars. He later went out with Optimus to attend a tour of the Earth and admire its sunset, and declined Optimus' offering of the Matrix, due to the fact that he doesn't know much about the Earth.

Eventually, Sam Witwicky discovered the Decepticons had the pillars and had planned for the Autobots to revive Sentinel as he was the only one who could use the Space Bridge. Once he arrived back at their base, Sentinel confirms of Sam's suspicions of the Decepticons' plans, but then betrays the Autobots by revealing his affiliation with Megatron, and kills Ironhide with his cosmic rust blaster. He destroys much of the NEST headquarters before bringing the pillars to Chicago and transporting hundreds of Decepticons that were concealed on the moon. Though Optimus tries to stop him from opening the space bridge, Sentinel compromises him, but lets him live in hope that Optimus would come to accept and realize the moral of what he was doing. He then orders the world to exile the Autobots, just to let them get killed by Starscream in their space ship, though the Autobots didn't actually board the ship. Soon after believing that the Autobots are gone, Sentinel Prime takes command over the Decepticons, betraying and pushing Megatron away by demoralizing him in the process, and oversees the final phase of the plan in Chicago.

During the final battle, once most of the Decepticons have been defeated by the Autobots, Sentinel battles Optimus, who has returned with the Autobots. Being more powerful and a more skillful warrior than Optimus and all the other autobots, Sentinel is able to more than hold his own against Optimus, Ratchet, Bumblebee, Sideswipe, and several human soldiers all at once. It is only when he is hit by several heavy-duty weapons and missiles from humans does he finally try to retreat, but is quickly re-engaged by Optimus. He is able to gain the upperhand, disarming Optimus of his weapons, then quite literally by severing Optimus' right arm. During the battle, Sentinel berates Optimus, blaming him for Cybertron's predicament, declaring that Optimus could never make the hard decisions necessary for victory. He then declares Cybertron for him to rule alone, preparing to strike the final blow to kill Optimus, but Megatron intervenes and angrily assaults Sentinel as revenge for taking command of the Decepticons, after being convinced by Carly Spencer, thus saving Optimus' life. After Megatron throws a critically wounded Sentinel to his knees, he proposes a truce with Optimus, but the latter, knowing what Megatron has in mind, refuses and instead kills him. After Megatron's death, Sentinel seemed to realize how far he had truly fallen, and pleads to Optimus to understand why he had to betray him and pleads for his life, but Optimus states that he had only betrayed himself, of his previous ideals of freedom. Without hesitation, Optimus takes Megatron's cannon and shoots Sentinel in the back and head, executing him. Optimus threw away Megatron's shotgun in disgust, and swore he would never forsake his new home of Earth.