Aetherplasm

The trick lives in the block step, a window most defensive cards ignore. Declare this 1/1 as a blocker, return it to hand before damage, and drop a fresh creature from hand into the same blocking assignment, paying nothing to deploy it. The fragile Illusion body is bait; the real card is whatever you held back. The substitute chumps or trades with the attacker depending on its body, the Illusion returns for another cycle, and you have developed your board at no mana cost during your opponent's combat. The discipline is that everything keys off blocking: it does nothing on offense, it asks you to keep a creature card in hand, and the replacement only blocks the one creature this was already assigned to, so you cannot improvise a multi-block out of it. What it rewards is creatures with combat-relevant enter-the-battlefield triggers or sturdy bodies you would rather not pay full price to deploy: walk a value engine into play already engaged in combat, banking the trigger while the Illusion bounces back to do it again next turn. The substitute stays put once it lands; the recursion belongs to the 1/1 alone, which is what makes the Illusion the engine rather than the payload. Few blockers convert the defensive step into a tempo and development tool rather than a damage-prevention measure, and this is one: a recurring summon-on-defense machine wearing a disposable body as a disguise.