Here’s a quick location you can stick into any dungeon or site-based adventure. Enjoy!
“Above the archway into this room is an engraved stone sign which reads ‘Do No Harm Lest Harm Be Done Unto You’. The room itself is large – at least 30′ square – with numerous pillars supporting the ceiling. The walls are coated in faded and patched murals depicting scenes of enemy races knelt in discussion, their weapons tossed to the ground.”
This room was designed with one purpose in mind: peace between otherwise unreconcilable enemies. Any direct damage caused in this room is immediately healed and inflicted on the attacker. For example, if a Dwarven Fighter hits a Goblin for 8 damage, the Goblin is instantly healed, and the Fighter takes the 8 damage, the wounds manifesting exactly as they have been dealt.
Add your own monsters into this room to taste. Perhaps this is the location where the Big Bad Villain explains his nefarious plans while taunting the players into striking him, or a large nest of Goblins invite the characters to join their Poker Night. This is a location is an ideal rest stop for the party – but it could also be a subtle warning that the party have unwittingly stumbled into a warzone.
Optional twist: The room fell into disuse after an adventuring Wizard used Invisibility and Ghost Sound to cunning effect, sowing discord between Orc and Hobgoblin leaders who had met to form an allegiance against the surface races. This tactic resulted in the death of twelve leaders and averted a major war. The Wizard was later found, killed and is now cursed to exist as a Phantom in the room, bound to repeat his trickery on all who attempt to use the room for peaceful discourse.