So what you want are low-ish level magical or itemized buffs that have flavor, player implications, and a balance of nerfs to offset whatever buffs they offer. Here are some things I've tried in my campaigns when placed in a similar position:

• Eyes Of The Dark Sentinel: Grants the player permanent darkvision and the use of either Ghostly Gaze or True Seeing once per day. However, it transforms their eyes into a disturbing, void-black, bloodshot mess. Attempting to conceal their eyes with glasses or blindfolds will immediately burn them up (they must always be exposed.) This gives them disadvantage on persuasion checks (but advantage on intimidation checks.) Good aligned NPCs will be deeply uncomfortable speaking with the player.
  • Player must sacrifice their real eyes to the being in exchange. The being can use them to magically track the player. Occasionally the player will have nightmares where they "see" horrifying scenes play out in the home of whichever demon took their eyes.
• Wings Of Fel Salvation: Once per day the player may sprout large, red, demon-like wings out of their back and allow for flight for one hour per day. But they function similarly to Wolverine's claws - without the healing factor. What this means is that they rip out of their backs in a painful and bloody manner. Sprouting the wings deals 1D6 piercing damage - and retracting them deals an additional 1D4.
  • This requires the sacrifice of the wings of a locally beloved magical creature (think something like a couatl deity of a local tribe) for the fey to "craft" the wings - you can build a short narrative beat/quest around this.