Yuusha Kei ni Shosu: Choubatsu Yuusha 9004-tai Keimu Kiroku

Sinned heroes will save this world...

Being sentenced to Heroship was the most severe punishment one could be given. Heroes would be sent to the frontline of a never-ending battle against the demon lords. Not even death could relieve them on their duty.

There was no end to prison sentence. Even if they fought for a hundred years, they would still not be released. Only by killing every last demon lord would they be pardoned for their crimes. And...that was no more than a pipe dream. However, things might be different for XYRO, Penal Hero Unit 9004. He accidentally meets a goddess...and she asks him to be her knight!

(Source: BookWalker)