Stream Level Effect

Hello! I have a TopDown project and I’m looking for a solution to my problem.

I have a dungeon with some rooms and I load thos rooms with a box trigger (that´s because I don’t want them to appear till the character pass into the door). The problem is that the room appears without any transition and it looks very ugly (:(). I’m looking for a smooth effect (or something like that) to make the room appears with a transition.

Thank you all!