I saw this movie when I was a kid so I don’t remember much about it. I do remember I the beginning if the movie that a group of adults or teens are running somewhere and they go inside a building. But a man leaves his bag outside and it seems some sort of rain falls on it and starts dissolving it. Inside of  the building they a find a head under a desk. There is also a naked shower scene. Somewhere in the building there is a monster that kills them. To get to the monster they have to crawl through a some sort of tunnel maybe a vent. The movie seems to end when a man stabs the monster and a baby is born out of it.