How to "blackout" a room in 3d game ?

Hello folks,

I'm currently working on a 3d dungeon crawler game where you can move the camera as you like.

As a concrete example I have a room with the dimensions 4x4x2. Now I'm looking for a solution to darken the room and put some kind of fog over it, so that the room is only completely uncovered when you enter the room.

Because the camera is freely movable, a simple 2D Fog Of War cannot be used.

Any idea what the best approach here would be?

My ideas so far:
- Create 3D Fog Of War, but that would be bad for the performance, because all rooms would be "darkened" initially.
- Create a cube inside the room with black/grey texture.

Greetings