Cracked Blue Brick

Every color of Dungeon brick has a cracked twin that looks almost identical to the real thing beside it. Touch one at speed, grapple onto it, mine it or land a projectile on it, and that block plus a spread of its neighbors drops out of the wall and shatters when it lands. They leave nothing behind when they go. The reason they exist is concealment: whole wings of the Dungeon sit behind cracked walls, and a Dungeon that looks small on your map is usually one you have not broken into yet.

The other thing they hide is a hole. Naturally placed patches in the floor frequently cap a rectangular pit lined with Spikes, and some of those are flooded so that climbing out is its own problem. Any fast approach along the floor is enough to open one, which is why sprinting down an unlit corridor is how people find them. Carry a Dangersense Potion. It lights the fragile tiles up plainly, and the only ones it will not flag are those that have been hammered into slopes.

When you want a passage rather than a surprise, speed is the tool. Run or fall onto them and the collapse chains outward on its own through the surrounding bricks. Riding a mount helps, since the larger hitbox catches more tiles per pass. Grenades are faster still and wipe every fragile tile in a decent radius around the blast, and Sticky Grenades let you put that blast exactly where you want it on a vertical wall. Expect it to be slower in multiplayer, where the collapse propagates in much smaller chunks. That is more digging, but it also means a hidden pit is far less likely to swallow you whole.

They can be crafted from plain Dungeon bricks at a Heavy Assembler, and the crafted ones are the genuine article rather than a decorative copy. They fall, they chain, and they still return nothing when broken, which makes them usable as a real trap floor in an arena or an adventure map. Their nearest relatives in behavior are thin ice and sand, though they match neither exactly.