There once was a powerful mage who summoned up and bound into his service a demon. Their contract was careful, precise, the wording had been carefully thought out by both parties, and the mage stood to gain much from the interaction, and the demon would have some time to wreck its mischeif on the side... But it was not to be. A great upheaval struck the land and the mage was forced to seal himself from time and the disruptions until things stabalized once more. Because of the careful wording of the contract, though, it was unable to purport the mischief it desired. At that point, it realized that for all that the original contract had been quite good for it, even if the mage had gotten all he wanted out of it, in the current state, it was most troubling, so it concocted a plan how to escape the situation. The demon took a portion of itself and created a new being, not subservient, but totally seperate. It carefully raised the new being to be intellegent, probably useful, but far less powerful than itself, and made sure that it could carry the 'burden' of the contract. Then the demon escaped from the world, and the contract, leaving its 'offspring' behind. The mage has returned now, to find this... Much less useful minion at his command, eager to serve him, as if not for the contract, it would not exist.
I'd really rather play the 'sacrifice' for this one, and probably 'GM' a bit too, as I expect there to be more to the game than 'The mage uses his new, willing toy.' Not that I don't mind running a good bit of that, I mean, he has been frozen for a few hundred years, might have some needs. The mage need not be male, or even have a constant gender, what with the ways of magic. The demon, and its offspring, will have an innate gender of the opposite of the mage, but are perfectly capable of morphing it. The only ruling I put on gender mixing craziness is that I prefer not to do flat f/f scenes (though will quite willingly do futa/f scenes) and absolutely no m/m, sorry. Also, torture should not be required, though the mage is certainly allowed to vent on the demonspawn. Variation suggestions will certainly be considered if one wants the scenario altered a bit.