Another thought:
The party is accosted by a Pit Fiend and a Balor who demand the party to be arbitrators in a (verbal) dispute they are having. The two threaten to do something vile to the party or those close to them if they do not cooperate and will follow through if refused.
If the party comes to a decision, the loser places a powerful curse on the party before vanishing. The winner remains long enough to grant a wish to the party, then departs with a cryptic "Good luck".
However, the whole thing is a setup and a sort of nickel-toss to see who would get things rolling (the curse is just to make the "game" more interesting). The wish, of course is tainted to cause ever-increasing havoc at the party's expense. The two fiends use a form of scrying to watch the evolving antics and surreptitiously influence the chaos. The pit fiend wants to coax the party into making planned, deliberate actions to ward off the ensuing chaos. The balor wants the characters to be spontaneous and emotional. Both want to see the party and those around them suffer. They will also actively use counterspell through the scry to foil attempts to undo the wish (and the curse), following it up with difficulty/tragedy to punish the players for attempting to spoil their "fun".
The idea is to put the characters through several encounters with escalating mishaps, some of which are purposely (and secretly) induced by minions of the two fiends.
Acute players can use the fiend's scrying to their advantage once they figure out they're being watched, even using it as a means to travel to where the two fiends are watching (using a ritual or item to open it as a sort of gate). The "game" ends when the party finds out a way to counter the ill effects of the wish or directly confronts the two fiends (and their assembled entourage) and defeat them (perhaps even possibly turning the two against each other or aiding one to defeat the other, thus allowing the survivor to "win" the game).
