I haven’t done any character building, but I did a little bit of text on the tacked-on pact for that warlock idea I had.
Once ever week, on Thursday around the witching hour, the pact-receiver will eat the entirety of the meal that the pact-provider sets out for her.
If the receiving party fails to finish the meal, her punishment is that next month they will eat the offered meal again in it’s entirety, plus an amount of food equal to the amount that the receiving party failed to finish the previous ritual meal.
Succeeding in finishing the meal, the pact-receiver gains good-boy/girl points, exchangeable for some paltry goods, magical or mundane, and with enough of them, you can summon the pact-provider in a combat, but that number is very high.
Failing the ritual enough times(up to the pact-provider’s whims, could be three failures, or thirty), the pact-provider may make minor changes to the pact receiver, be it a simple change in hair color, some new clothes they must wear during the ritual feasts, or maybe they will make their little pet pact-receiver wear a collar, denoting who they belong to.
Use will saves(something that the player is hesitant to eat, or out right hates) to make them push through the pain and eat the meal. The offered meal is up to the DM, but remember that fiends enjoy watching the ever dwindling hope of success being just out of reach of the mortals.
Use constitution checks, if the meal seems to be more than what the pact-receiver can stomach, with an appropriate difficulty level for how the scene is proceeding, after failing three times in a ritual, they are stuffed to the gills, or will burst from one more bite.
While it’d be fun to go full tilt at the start, lull the player into a false sense of security that they can easily handle the challenges. Then turn that smugness around on them, dangle the win just out of their reach once they start to grow complacent.
As for the point store, that is up to you, personally I think secretly giving a bit of a cursed magical item that does what they want, but gives them odd quirks or compulsions(such as more frequent burping, fuck up their stealth checks with comedic timing; or have them compelled to drink anything in a bottle before them, this is a fun way to get at the well prepared travelers and their stash of potions, antidotes, and alchemist’s fire)
Be lenient with how many failures the player must make before their fiend master starts to toy with them, while it is ultimately your prerogative, you should get consent from your player for anything that might embarrass them, don’t be a dick-dm a force them into your personal magical realm.