Echo Wall

An invisible background wall that still does the two jobs a wall exists to do: it stops enemies spawning behind it and it counts toward valid housing. Nothing renders unless you are wearing Spectre Goggles or standing inside the range of an Echo Chamber.

That combination is the whole point. You can put a legal NPC room in what looks like open sky, run a spawn-proofed corridor behind scenery with no visible seam, or hollow out a hillside without the wall giving away that it was touched.

You cannot see what you have already placed, so mark the boundaries with something solid before you start or you will lose track of where the coverage ends and spend ten minutes hunting a gap. Echo Coating does nearly the same thing to an ordinary wall for less, so reach for this only when you want an entire surface gone rather than one specific piece hidden.