Well, I might sound unoriginal here, with all the hospitals and such, but here is another horror house:
Back-story: Giant complex devoted to healing attracted less-than noble people. Eventually remade as a madhouse. Fifty years after it and the community that surrounded it underwent a "mass lockdown", the house still runs itself. People can still come in. No one comes out.
First off, the PC's go inside this lovely place without much thought. They need to find records about a individual, one who was supposedly related to the main plotline. They walk inside the main lobby, and as it turns out it looks quite good for a place thatís supposedly abandoned. This is a setting where magic is vaguely spread out all over the place, so lights that are some how working in a supposedly haunted location did not surprise the players.
The players enter one of the basic sections of the building, and find out that the building is actually in a strange condition. While most of it is spotless, spots on the walls, ceiling, and floor look as if that, yes, this place has been undergoing the same wear and tear as any other abandoned building. Some pretty white spots on a otherwise grimy and water-damaged wall resembled hand prints from a small child. The bones of a strange creature (Like a cat, but "stretched out" as one PC with the right skills figured out) was laying on one of the grimy spots on the floor.
Eventually, after finding that these spots grow in size as the players go deeper into the complex, they find something interesting. To be exact, what is seemingly a human jawbone fused into the wall, in a position that would suggest a person with there chin up was standing there. The bone flawlessly merged into the clean, untouched spot, and attempts to remove it or the clean spot ended without success. Each and every one of the five video cameras the party brought did not work, seeing as they where out of battery somehow.
Eventually, as they go on in a quest for the "center" of the complex, they find that, rather than water-damaged and moldy, the damaged spots are taking on the look of something that was burnt. Some spots where still smoldering, with many having wisps of smoke coming out. One room that was wholly burnt caused the PC's to be forced to make several checks based on endurance to avoid taking in to much smoke. The one PC who failed that test got a nasty little trip in which the unnaturally clean walls where of bone and the blackened parts red and oozing foul ichor. She snapped out of it eventually.
They soon figure out that this place is bigger than it was outside, and that they should have long since made it to the center of the building. There attempts to find the map's for the floor plan fail, as each one that they find on the exits are burnt and smoking. They also find more body parts fused to the walls, including a rather fleshy arm, several finger bones, a leg, and what was seemingly the backside of a man. Now, most of the building has switched back to a unnatural clean.
As if to make up for the clean earlier, the building is almost entirely ruined in the next few hallways. It's condition now takes on the worst of the former two versions, with whole hallways consisting of warm, blackened tiles and warped wood almost entirely covered in warm, sooty water. The floor nearly falls under them a few times. The only piece of human they see is a single fleshy torso, without arms or legs that was erect in the center of the hallway. The PC's were relucent to go past it, but they eventually did.
Eventually, they manage to find a hallway with actual rooms. The first one they find is almost entirely normal-ish, besides one little thing: all objects are fused to the floor. Carts are somehow immovable, chair legs are few inches inside the floor, ect. All are immovable. Windows, as it turns out, are not immune to the test of time, as they find out as a PC, who was knocking into things attempting to make them move,punchs one and gets a arm full of glass. Looking outside, they find out that they are only up to the second floor on a fifteen floor building.
Eventually, they get to a place that might have been the lunch area. Mostly clean very few spots that seemed dirty. However, one of the PC's got a strongly high spot check, and got this interesting message:
"You find a strange crease in the floor."
Naturally, the PC pokes it, and subsequently opens suddenly to reveal a mouth. That screams.
Thatís the point where the floor, walls, and ceiling deformed in horrible ways to show moving, screaming bodies, with arms, legs, and things not native to humanities body starting to flail in every direction. The walls themselves where pulsing from the movement going on inside, and the same aged spots from before quickly spread across every surface like water. To put it bluntly, the entire room was seemingly a horrific melding of deformed metal and flesh, and it hated the PC's. They quickly ran across the room, seeing that the door they came in through had sealed itself and became one with the pulsing room, and managed to take only a little bit of damage.
They quickly found that the building past that was still a pulsing horror, but not nearly as bad as the first room. However, things like chairs and desks became much like the fleshy abomination that they saw before, and they quickly learned that the difference between metal and flesh ended the moment they walked into the building.
After some encounters with some formerly stationary objects, they manage to actually get there way inside where the files where kept and got the files about the guy. Naturally, they wanted the heck out, and they found a exit near the room.
After nearly sliding on the wet stairs (that had a tongue-like consistency), they start to hear the same screaming as the horror room from before, but louder. After a few stretches of perfectly clean whiteness, they find a door that said "EXIT". The screaming only got worse as they got closer to it.
However, the phrase under that stated, in blood-red letters actually painted in still-wet blood "A MONUMET TO ALL YOUR SINS", misspelling intended. They walk into the room, and what they saw shocked them, which was surprising considering what they had gone through.
The room itself was massive. Rather than having the same "hospital white" scheme as the other parts of the building, this cylinder-shaped room was almost chrome like. Also, it was massive: it was hardly large in width, but the top was unseeable to them. They also figured out why, out of everything they saw, there was not a single head anywhere in the building. Or, for that matter, why the screaming stopped.
The entire room had heads coming out of the walls, with the whole thing jam-packed with heads each flawlessly sealed into the walls. Each one was looking at the PC's, without saying a word. The players, now slightly tired, where horrified, and on the PC's asked "Why".
At this, one of the heads gave a little chuckle. Soon, others began to laugh, and eventually every head was guffawing at the player's question. Soon, droplets of blood rained down from the unseeable ceiling and the entire structure was shaking slightly, a organic motion for something otherwise wholly metal. Without much more talk (Albeit one of the PC's nearly broke down), they walked outside, past the freakish doors.....to daylight. They actually had spent less than a hour in there, besides the fact that night had passed by since they where inside. The clincher was when one of the PC's looked at the building behind them. It was a normal exit, and the door had a message spray-painted on it.
THERE IS NO WHY, TRAVIS, THERE IS NO WHY
One of the players nearly broke down there (the player who asked the question's charecter was named Travis), while two others needed to use the bathroom. I feel...accomplished.