2D game in 3D Worldspace (Metroidvania) Room visibility // Field of view?

I'm currently working on a Metroidvania game with a friend of mine.

Im struggeling with the following Problem:

I wan't that the player is only seeing the room he's currently in and all the other rooms or what lays "out of bounds" shouldn't be visible to him. But i don't really know how to tackle this particular problem...

Has anyone any tipps or tricks to achieve this "effect"? And what is this effect called? Greetings from Switzerland (Btw. sorry for my english and my bad paint drawing ^^)