Interesting. Just a few comments, I'd like to make. First, I take it the perfect ink whether used or not will vanish when they die/return to hell. That in itself could be a lever for another deal. Second, I think that one of these critters would make an excellent hell's attorney type of a character. Third, it seems odd to me that the souls of the dead children (and Mephistori) are somehow in the devil's bodies. Does that mean that the devils wanted to return to their respective families? Like I said interesting. I have a use for this.
22.) Arkanik's Doppelganger Prosthetics
Arkanik was a wizard researcher who spent most of his days studying shape changing creatures like doppelgangers, changelings, mimics, and so forth. He was maimed from a childhood incident and was forced to live with a painful prosthetic most of his life. Eventually became interested in a mutal relationship with less intelligent shape shifters. And so the Doppelganger Prosthetics were born.
The prosthetic is actually a separate shape changing creature with very little mind. Initially it is sculpted into the limb it was meant to replace and attached. As it's owner uses it, as the said limb, it strengthens and better acts in that capacity. The limb must be separated daily, fed, and cleaned (it has it's own waste disposal system). The limb will develop it's own 'personality' with it's own sets of likes and dislikes. If treated well, it will act well. If treated poorly it will betray or leave it's owner. Overtime the prosthetic will learn it's owners disposition and emotions and act in compliance or rebellion to that emotion depending on it's treatment. This 'pet' of an appendage might even seek retribution upon it's master's killers if such an event should occur.