Sentinel Prime hails from an older time, when Cybertron was the galaxy's crown jewel, and the Transformer race was full of promise and potential. The only known remaining Prime, he personified what a leader should be: wise, compassionate, and a more than worthy warrior. Revered by both Autobot and Decepticon, few would dare stand against his shield and Primax Blade.
But when Sentinel was presumably lost forever on board the Ark, Optimus Prime, his hand-picked successor, was left to fill his void. The loss of Sentinel signaled the end of Cybertron's golden age, and the planet spiraled deeper into war. Some who recall Sentinel's reign bitterly remember when Transformers were masters of their fate, rather than scattered to the stars and living as exiles.
Sentinel Prime (c) Hasbro