The material below is Open Game Content from a 3PP Source
An extraplanar being from some infernal realm approaches the PCs. This could be a demon, a devil or whatever is proper for your campaign. This being claims, quite sincerely, that he has decided to turn over a new leaf. He wants to do good, and wants the players to teach him how to go about doing so.
The creature is quite powerful, and can withstand any attacks the PCs may hurl at him. If they decide to accept his help, they run into the problem of what to do when people see them spending quality time with a diabolical fiend from a dark nether realm. Friends will attack the demon, believing that they are helping the PCs. The demon will defend himself and a lot of people will die until the PCs find some way to remedy the situation.