Position: Middle to end
Everything you read can be fodder for a scenario. In this case, Nightmares and Fairytales has a great sequence with nuns guarding an unspeakable horror that was the inspiration for this room.
What you will need for this is a room with a large door, one that can be made to look imposing. A painted plywood covering can mimic the look of a heavy wooden door, dark with age, particularly if two-by-fours are fastened to it. Don't forget the chains, locks, and other impedimentia that will keep the door from opening more than enough to stick a hand through. For safety's sake, consider a fake wall and door designed so that no one can get an arm caught when the door is slammed, as may happen at some point in the performance. Light the space on the far side with reds and black light, or strobes. Finish decorating the main room with cobwebs and dust and religious paraphernalia, particularly ornate crosses.
The idea is that you have religious figures, monks or nuns, praying to keep the guests of the house safe. Prayers featuring ornate Latin laced with "Vade retro, Satana" ("Get thee behind me, Satan,") are good. If you have a guide, have them wax eloquent about the dark forces behind the door— only to break off as the door starts to get forced open. The religious figures break off their prayers and chanting immediately and try to force the door closed (but not too hard; remember that we don't want to shut the door on anyone's arm!) as red light and fog start to pour out of the opening. Monsterish bits (zombie arms, tentacles, whatever you have on hand) start to force their way around the door.
Whatever it is grabs someone and pulls their arm through. That person starts screaming; another monk or nun runs off to grab a crucifix. If you like, have the victim pull free and display a bloodied arm. Either the religious people lose control and start shouting for the guests to run or have them push the victim through the door— because once the monsters have bloodied you, you will become one of them.