Cathedral Membrane

A 0/3 with Defender reads as a free trade: send any attacker in, watch the wall die, move on. The death trigger is the trap. If something kills it during the combat in which it blocked, it answers with six damage to whatever it blocked, which means the attacker that "got through" by killing it gets killed in return. The interaction lives in the combat damage step itself. The defending creature can take lethal damage at the same moment it deals (or fails to deal) its own, then die and trigger; the six points come back once it has died, a delayed retaliation rather than a damage-prevention shield. The strategic axis is bluff. As a single blocker it can only stop one creature per combat, so its value is not the block but the threat of the block: an attacker has to decide whether the body it is swinging is worth losing to a wall it cannot profitably hit. The Phyrexian mana clause lets it come down without white, paid in two life, so even a deck light on its color can leave the threat of it open. And it inverts the usual defensive instinct: most walls want to survive, while this one does its real work only by dying on the block, which makes any combat trick spent to save it actively counterproductive.