Enterthedave.bsp is a map that can not be found in the actual Nightmare House 2 game, but it can be found by accessing a folder that stores the Lost Files for Nightmare House 2.
At the start of the map, the Patient begins his adventure by falling through the crumbling ground. He has his flashlight from the start and he will see a floating text in midair that reads "This map was made by Guessmyname, planned to be some kind of secret map you can access via chapter 5 in NH2. We planned to give some kind of a backstory about the mannequins using this map. We had to cut it down because of the deadline". The Patient must cut his way through many obstacles without any weapons that would have been useful for any type of assistance.
He is able to find some flares in the map as a light source so he does not have to drown out the power in his flashlight. As he tries to find his way around, he stumbles inside a bright room that holds a mannequin in it. The room's light flickers off, forcing the Patient turn his flashlight on in order to see through the darkness. However, after he turns the flashlight on, it is noticeable that the mannequin disappears. The room leads to a staircase with another mannequin waiting for the Patient at the top. Once he reaches the top, the mannequin disappears too.
As he keeps took action through the dark area, he finds a second staircase that leads to a room with two mannequins inside it, one that is lying down on the floor while the other one faces the Patient as if it were gazing continuously at him. The mannequin staring at him disappears like the other mannequins. He proceeds through an opening in the wall leading to an area covered in debris and trashed with crates and shelves. He finds another room piling up to the ceiling with debris along with two mannequins facing the Patient, fastened in the pile.
As he makes his way around the debris, he stumbles upon another room with a mannequin standing in the corner of the room. Once the Patient dwells close enough to the lone mannequin, a whole multitude of mannequins appear in the room, surrounding the Patient and pass from his sight. He exceeds towards the doors for exiting the room that leads into a district of darkness with an unaccompanied mannequin in the center of the darkness. As he advances towards the mannequin, the mannequin is given a fleshy appearance and the Patient is telekineticly forced towards before he loses consciousness.