I came up with a concept for a scary monster for a sort of set piece situation inside of an abandoned house or bunker with thick walls.
It's called a Waller. The thing lives inside of walls, skittering around and thumping it a lot. If you get too close to a wall for too long, it'll break through and devour your fucking skull in a flash before drawing back in and vanishing.
The idea is that it's not really seen, only in flashes, and is mostly heard, seeming to be somewhat insectile, much too big for being inside of a (still pretty thick) wall, elongated, and FAST.
Combine it with concepts like a narrow passageway to race down, holes in the wall you need to leap through fast as possible, thinking you've killed it by collapsing part of the house only to still hear it later, and so on.
I'd like to try it on my players someday if I can get a few of them separate, desperate, and mostly disarmed, and a good ways into the structure before the thing starts to attack.