Active Stone Block

Two blocks, one mechanism. The active version is an ordinary solid block in every way that counts: you walk on it, it stops liquid, it blocks light. The Inactive Stone Block is the same block pushed into the background, still visible but empty, and players, water, light and items pass straight through it. Wiring a trigger swaps every one of them the wire touches from one state to the other, and that is the entire feature. The Wire has to physically run through each block you want to flip, so plan the run before you build the wall. Inactive blocks also cannot be placed against each other with nothing solid nearby, so the normal build order is to lay them all down active and flip them afterward.

The best known use is a doorway of three stacked blocks wired to a Switch or a Lever. No enemy can ever open it, and town NPCs will not wander out through it either, which is the standing complaint about ordinary doors. Wire a Blue Pressure Plate to each side and it opens as you step on and shuts again behind you the instant you clear it, since those plates ignore everything except players. That is enough to keep zombies outside during a Blood Moon. Two of them in series gives you an airlock, which is the only reliable entrance for an underground base. One catch: a stone door does not satisfy a house's door requirement, so an NPC room still needs a real one somewhere, even if it only opens into the next room.

Furniture sitting on blocks that go inactive is cut loose and drops as items. Blocks placed alongside them stay hanging in midair exactly as if you had mined the support away. Whichever state a block is in when you mine it is the version you get back, so you can convert a whole stack by placing it, flipping it with wire, and picking it up again. Inactive blocks mine normally, and explosions free them too.

An Actuator does the same job to any block at all, which makes it the general tool and this pair the specialist one. The difference that bites is support. Turn one of these off and it drops into the background entirely, so nothing can be built against it, while an actuated block still counts as a block for that purpose. What the stone keeps is immunity to biome spread, so a structure built from it never turns corrupt or crimson around you. Unwired, it is simply stone that will not change color.