So a player used 'wish' to become a Pit Fiend. What is it, and what happens next?

[Gildedragon]

But if you want plot... What's the party's average alignment?

Quick and easy idea:
He becomes a pitfiend, ousting someone else in line for the powerup
He is a ruthless fiend and is upsetting the order of the Hells
A lesser demon enlists the party to help set the infernal machinery right