Ideas needed for BBEG using devils

[Blatant Beast]

Naomi Novik, in her novel Spinning Silver, has an interesting take on this trope:

The deal with the demon, for the second son to assume the throne, was made by the second son's mother, before the the spare prince was born. Of course the mother offered up her son's soul as part of the cost of the bargain.

The Demon, therefore, initiated the plan to kill the the first born king, without the approval of the beneficiary of the plot, (the second son).

The demonically supported Prince, hates being king, and would gladly be rid of the demon, but the demon inhabits his body, (can take actual possession of the body, and does so at night), and the demon inflicts terrible punishments upon the young man, to compel compliance, and has done so for the prince's entire life.

This would add an extra moral dimension to the setup, and adds a potential win condition of the PC group being able to break the deal for the King, and free them of the demonic influence.