The Blood Service

Aaron's no super soldier. He's just a convict—a Capital criminal.
They gave him a lifetime sentence at thirteen years old, so that he could toil his remaining days in the Empire's distant galactic colonies. But not every planet the Empire has colonized is empty.
Aaron doesn't need cybernetics to hear the big beasts battering away at the colony wall. These creatures dig day and night, two tons of leather and bone sending violent tremors through the dry soil. They're coming, they're coordinated, vicious and relentless.
Even all of the technology, weapons, and super soldiers can't beat politics. Orders down for the colony garrison to depart, leaving them to the vengeful whims of their alien host. So the clever commander comes up with an idea. They may not be soldiers, but the Capital convicts are strong, united, and just desperate enough. They offer Aaron and his friends a devil's bargain:
Serve a tour, go free.