It's easy.
- Cain is up for an award, or a promotion, or he's trying to get something done that requires clearance of some kind.
- A routine personnel check shows that a lot of Cain's records are either incomplete or redacted from a higher source? What source? Why are Cain's files incomplete? How did he apply to be a Commissar in the first place.
- Cain is hesitant, but eventually puts a name to his birth-world. Again, as a routine check, some Admins look up the planet. Turns out like...Two months ago - or whatever a short time is - the planet was hit by an undocumented Tyranid/Necron attack (because it's Cain, it's always Tyranids or Necrons), convenient, no?
- Cain panics when he realises that Zyvan wants to take his units and go investigate. Cain deserts his post not being able to face his nightmare and his destroyed planet.
- Cain is caught and put into a Penal Legion for desertion and sent to the planet anyway. Uh oh. What wacky hijinks will Cain get up to this time?
Being in a Penal Legion is bad. Really, really bad. These guys don't give a flying s* about Cain's reputation (unless they could make fun of him for it), and weakness is most definitely not allowed. Hell, the Penal Legion Commissar/Handler can get killed, and through some fast talk, huge coincidence and amazing skill (which Cain still says he doesn't have), Cain puts on the Handler's sash and cap and marches his Penal Unit (there's a joke there...) to victory.