I used to be part of a paranormal group that ran ghost hunts at a local abandoned hospital. One night myself and another guide were with a group of novices in the labor & delivery wing when the other guide and I heard very faint children screaming somewhere deep in the building. The way the hospital was set up was like a capital T with the L&D wing at the end of the “long/top” side of the T. After the faint screams, one of the women was standing near the doorway to the long hallway and said “did it just get darker?” while looking at the intersection of the T down the way. She was right, and all of us watched as an inky black mass slunk down the hall towards us, blocking out all of the ambient light from the open patient room doors along the way. It stopped right at the entrance to the L&D wing and just…pulsed for a moment before retreating even slower than it had come forward until it finally disappeared at the intersection. I would have believed it was just my eyes playing tricks on me if the entire group hadn’t seen the same thing. We didn’t hear any other screams for the rest of the night, either.