D&D 5E Need advice on how to reward 1 character but not whole party.

[outsider]

Generally, demonic pacts give the reward upfront(the rewards probably were given during the character's backstory), and the "payment" required for these rewards is left somewhat ambiguous. Usually, you're not off the hook for doing one thing for them. The demon should keep using the character in worse and worse ways.

Basically, the character in question should start feeling like Lando as the campaign progresses.

https://www.youtube.com/watch?v=A-sA-NqMlvI

The character in question should be looking for a way to get out from under the demon's thumb now, and if he's not evil in alignment he should probably be looking for redemption as well.

 

[discosoc]

I feel bad for your other players. Few things are more annoying than secret plots between the DM and another player, especially when the negatively affect the rest of the party. That's the kind of stuff that can break a group up.

 

[outsider]

I feel bad for your other players. Few things are more annoying than secret plots between the DM and another player, especially when the negatively affect the rest of the party. That's the kind of stuff that can break a group up.

I do agree. To keep the campaign running smoothly, I think it would be VERY important to move the character from "willing participant in demonic plot" to "actively rebelling against demonic master". The character should probably be forced to confront the reality of what he did. As an example, if there's an npc the player liked, that character could die as a direct result of aiding that demon. Something important to that character should be lost.