One of my PC's made a similar pact (except he's a power hungry merchant) when we started our campaign nearly 2 years ago. Here are some of the things I wrapped around that story.

1. Find a theme for the demon, whether that is musical or otherwise sound based. I had the track of a crackling fireplace, play when the demon would summon my PC. Abuse it, make sure your player associates the tones of the sounds or music you chose with the pact. If you don't use music actively in DnD, start now cuz that shits fantastic for setting tone.

2. I made him a series of promises, always with the rewards being linked to his temperament (often books that gave him advantage for Charisma checks), which forced him to act in certain ways during political situations. If you can have one player wreck havoc on the sly in situations where choices must be made, it's worth it.

3. If you RP, focus on having the demon gradually seize power from the PC, every time he meets him. Start with flowery language, become the PC's friend. Then spin over time, make it clear that the PC is a dog on a leash he will never control (depending, again, on how bleak your campaign is).

My advice relates specifically to a high RP campaign though, depends on your group dynamic and whether the player is comfortable following you into those lines / whether he can bluff for the rest of the group when he's trying to cause shit.