Now you have to live the rest of your life knowing you're going to die someday and disappear forever.
Optimus Prime is the leader of the Autobots. Long ago, he was the humble leader of Cybertron's science division, which studied the AllSpark, and were protected by Lord High Protector Megatron's military. When Megatron was corrupted by The Fallen, Optimus discovered that he was the last of the Dynasty of Primes, hidden away when The Fallen destroyed their lineage. In response to Megatron's attempts to seize the AllSpark and conquer other worlds, Optimus assumed his proper title and rallied the Autobots to stand against the Decepticons.
As obsessively as Megatron strives to achieve his objective, Optimus is equally as dedicated to stopping him, even at the cost of his life, his world, and—if necessary—his own people. It was on his order that the AllSpark was jettisoned into space, dooming Cybertron to a slow death and his people to a nomadic life. The sacrifices he has made weigh heavily on his spark, but they must be done, for there is no other choice. He knows the ambitions that lurk in Megatron's spark, and they must never see the light of day. Lives must be kept free from the threat of Megatron's tyranny, no matter the cost. This obsession with the defeat of Megatron has robbed Optimus of some of his early idealism. This has created a certain ruthlessness beneath his otherwise honest and caring personality. Prime is not one for second chances and is willing to consider any action that might lead to victory, short of directly sacrificing those he cares about.
He can be seen among the crowd of Oasis civilians during the final battle with other characters.