Prototype is one of the seven short films comprising the anime adaptation of the Halo universe, Halo Legends [1][2].
The story follows a Marine sent to a distant planet to destroy a prototype powered armor, but when he arrives, he finds the planet is still inhabited by Humans who are under siege by the Covenant Empire. The suit is their only hope for survival, forcing him to disobey orders and use the suit to help the civilian populace.