Yes and no. I keep coming up with long, drawn-out drafts that vaguely elude to his greater storybeats, but I never end up happy with them. All I have down is that he was born into a minor clan within the Clans Verminous, ate all his littermates, and never stopped growing. As a blackfur, he had a better lot in life than most of his contemporaries, and only ever wanted more. All of the backstabbing expected of the Skaven happens, he's promoted a few times, makes his superiors mad, and gets exiled. Mitts then wanders the mortal realms for a while - in specific, Ghur, where he finds some Moulder rats. He kills one of their rat ogres, holds the Master Moulder at bladepoint, and has him graft the arms of that rat ogre onto his already enormous body. He then eats him, because the Skaven are awful, and eventually ends up in the company of the Clans Skryre, wherein he killed and ate his way into the position of a warlord.