The movie is basically about a group of students, I think studying psychology, who finds a book of one of the past professors.
Basically research said: do this ritual and they get drawn to another world. However they did this and nothing happened, but when they see a reflection, they see the reflection of this professor who's book they found. Each time this happens, they go into his world and repeatedly die or something.
I remember this was in a college and one of the members was a ballerina who got taken in while she was practicing in the dance room full of mirrors.
I also remember that this professors research they found, he is the one that pulls them in to his world where he can like imagine whatever he wants. Also his world is really just a house where he lives in and his guests are these people that do the ritual. Eventually they beat him by bringing their current professor in, overwhelming the world with his own. However in the end he stays in that world on his own.