This seemingly mundane brass door knob can be inserted into any wall and turned to the right to create a 3′ wide door through the wall that swings inward cleanly and easily, as long as it is not otherwise obstructed, regardless of the thickness or material of the wall.
Turning the knob to the left detaches it from the wall, at which point the door swings swiftly closed if it is open. Creatures in the path of the closing door must make a dexterity saving throw. If under 12, they take 2d6 bludgeoning damage. If under 15, the side of the door on which they end up is determined randomly. The door then returns to seamlessly to being part of the wall.
After use, the wall knob requires time to recover dependent on the material of the wall. 2 hours per foot of wood, 4 hours per foot of stone. The wall can be no more than 25 feet thick. The DC of Stealth checks to open the door unnoticed by people in sight of it inside is equal to the thickness of the wall.
Yes, it can be a bummer for the players to bypass large sections of dungeons using spells and magical items (see the Haven post later this week for an example of that), but they’re going to do it anyway. As Detentions and Dragons is fond of saying, good items give the player greater agency, and I think this does that in an interesting way. Thanks to my wife for the fun idea.