Starchaser: The Legend of Orin (1985)

Set on the subterranean Mine-World, a band of human worker are treated like slaves under the power of the evil overlord Zygon until one, Orin, unearths the hilt of a mythical sword that only he can master. Escaping the planet, he runs into the rogue smuggler Dagg and a pair of helpful droids and the princess, who all team up to return to the Mine-World with a plan to defeat Zygon and free Orin's enslaved people.