Penal legion

Penal Legions are regiments of the Imperial Guard composed solely of criminals. Instead of serving a very long sentence these men and women have been given the once-in-a-lifetime opportunity to fight and die for the Imperium, thereby wiping away their past sins and allowing them to achieve the Emperor's peace. Needless to say, Imperial commanders tend to use Penal Legions as an expandable resource, even more so than normal regiments.