Opal Guardian

Cast on its own, this sits on the battlefield doing nothing: no toughness, no power, no board presence, just a triple-white enchantment waiting for a trigger that the controller cannot supply. That is the whole gambit. It punishes the opponent's deckbuilding by lying dormant until they cast a creature, at which point it stands up as a 3/4 flier with protection from red. The design reads as a deliberate inversion of the usual creature-enchantment relationship: rather than a creature that protects itself by becoming an enchantment, this is an enchantment that arms itself off an opponent's tempo. The catch is that a control mirror or an artifact-and-burn deck never wakes it up, so its body is conditional on facing the very strategy it is built to blunt. The protection from red is the tell about its intended job: a flying wall that red removal cannot kill and that trades up against red's ground creatures, slotted into a white deck expecting to face aggressive red opponents. It is a sideboard-style answer printed at the main-deck level, an enchantment that politely refuses to commit until the matchup justifies it, which makes it as much a wager on the metagame as a card you cast.