I'm GMing a 5e Dungeons and Dragons game, set in a Forgotten Realms-like world.
I want the party to enter a room and see something like a lever, that appears to be a reversible mechanism but once activated cannot be deactivated.
As there is a high risk of death due to this mechanic, I don't want the players to feel cheated with something as simple as "it's broken" or "it's locked in place", only to have me stop them from succeeding on any repair spell or strength check.
It's important to the story and the dungeon that this is not something they can reverse easily. If they manage to come up with a solution to whatever I decide to implement, then that's fine, but I don't want this to be designed with any solution in mind.
TL;DR: I need a mechanism that appears two-way, but is not reversible.
There is a room with a painting. In the painting is detailed the characters minus one (player or NPC, I don't know yet) as well as an unlocked and open door. There is no way to open this door in the room they are currently in.
Also detailed in the painting is an activated mechanism similar to the one found in the real room. The idea is that activating this mechanism will "switch" the room and the painting, opening the door but effectively trapping the character in oil pastel hell.
I'm planning on making it very clear that this will happen throughout the dungeon (this is nearer the end), and I don't want the players to be able to simply finish the dungeon, come back through, and then pull the lever again to bring the player back.
I also want this to at least seem fair to the players, rather than me railroading them into a death.
I'm considering simple having the mechanism disappear from the painting once they pull it, indicating that the mechanism is no longer affecting the painting, but I would prefer something physical that they can investigate and at least try to 'solve'.