I was running Castle Ravenloft back in 2e, and the party had just recently been drawing from a Deck of Many Things. The party's cleric had drawn the card "Defeat next enemy singlehandedly to go up a level."
The next enemy turned out to be a Shadow Demon. Upon seeing it, the party broke into a conversation, egging the cleric that he could easily win against the creature (the cleric had just recently acquired a mace of disruption). The demon, realizing it would be easier to defeat one foe at a time - and a chance to take out the party's primary healer to boot - announced aloud it would willingly battle one-on-one with the cleric, and not attempt to flee the battle.
The group then blocked both exits out of the room and the duel began. The shadow demon opened up the combat by plunging the room into magical darkness. Each round, the cleric had to choose the square he was going to attack before he even made a roll. To further egg the cleric on, the shadow demon would occasionally lash out at one of the onlooking PCs (who were barring the doorways yet still inside the area of darkness); they couldn't retaliate back without breeching the terms of the cleric's duel. It was a tense fight in which the cleric used just about all of his healing spells on himself just to remain standing. He eventually won, but only barely - being at only 7 hp and catching the Shadow Demon just before it attempted to renege on its promise not to flee the combat by teleporting away, since it had been badly injured itself.
