The Traitor Guard
You regiment has been misused and abused by the Imperium during yourservice, stretched to the breaking point, and after a deployment in which you leanred things you should not know, you were slated for execution by the Inquisition. It should have been the routine destruction, but it wasn't, and your regiment rebelled agaisnt it, freeing itself from the inquisition and the rule of the Imperium. Now renegades, wether you have chosen to or not, you have attracted the eye of chaos, and it has great things planned for you.
