I have the idea of a person being given a gift of the fiends by wish the natural ability of casting spells from their own effected body. Be it a disfigured pair of legs, a strange tail, or even a demonic arm…it can be anything with relative traits of a demon or devils influence.
This is more so a template to place on, which doesn't grant any other benefit but the soul purpose of allowing a deal making character, or a character who slew a demon, the ability to take on demonic power traits.
This allows them to take on a domain progression of choice with which can be discussed with a DM of choice or the council, or on the request forum.
My first test is with using zeo roseblade as a test subject for it. no apparent bonus or negatives other than the given capabilities from the stat bonus's of tiefling, and then giving the placing of fiendish gift to progress with spells based on the power of his arm.
which I will place out in progression here.
1-2 HD:mage hand
3-4 HD:cutting hand
5-6 HD:spectral hand
7-8 HD:helping hand
9-10 HD:force claw
11-12 HD:interposing hand
13-14 HD:forceful hand
15-16 HD:Grasping hand
17-18 HD:Clenching fist
19-20 HD:Crushing hand
In a sense it's like gaining a devil/demonic domain ability at 1/day with your level equaling your caster level. In which, my request would be the use of this mechanic.