The Nightmare Lock is a unique Adventure Mode structure used to trap the person sitting on the Nightmare Throne. It looks like a tiny version of the Wooden Thing with spikes surrounding it, and can only be found at the epilogue of the Adventure Mode beside Maxwell. Also, a Divining Rod is required to unlock the lock. After unlocking, Maxwell (or whatever character was put there last) will be released, and the player's character will take his place on the Nightmare Throne. Back in Sandbox Mode, the player will become Maxwell but will keep the items they had when they activated Maxwell's Door.
- In older versions of the game, the Shadow Key (which shared the same model as a Purple Gem) was used to free Maxwell.
- If the player spawns a Nightmare Lock in a world and tries to unlock it with a Divining Rod, the game will crash.