Darkness in the certain parts of the scene

I'm looking a way to implement darkness to my 1st person horror game demo but only to the certain parts of the game environment, like tunnels and rooms. When the player enters those areas the visibility is reduced and when the player leaves those ares the visibility is restored.

I was thinking about trigger colliders and fog setting changes but it will not give the best results to my game. Any other ideas? Post-processing?