Where’d everybody go?
As one of the characters enters this seemingly empty room, he disappears. Rather, he turns invisible, but to the characters behind him he’ll appear to have disappeared.
The same thing happens if anyone else enters the room. They all turn invisible, including anything they’re carrying. If anything is tossed into the room, it, too, turns invisible.
The room is used to store treasure, and there are a couple of invisible treasure chests hidden inside. Additionally, there is a magical beast lurking in the room, one for whom lack of sight is not a hindrance.
Love it. A perfect monster would be something that had truesight, and could transport to the ethereal plane as well, in case the PC’s could see invisible things as well.
Now that’s just mean!