These are monsters that hide in the environment (walls, floors, ceilings, etc.) and never fully appear until you walk close to them, at which point they suddenly lunge out and try to grab you. Can be extremely annoying, especially when they guard narrow ledges, as you can't target them until you're within their attacking range. Compare Goddamned Bats and Demonic Spiders, depending on what the particular monster is capable of. See also Chest Monster for enemies that hide by disguising themselves as something benign. Examples of Wall Master include: