Opal Gargoyle

Two mana buys a permanent that, on resolution, does precisely nothing: an enchantment with no static effect, no triggered value, no immediate footprint. It waits. The single condition that wakes it is an opponent casting a creature spell, at which point the enchantment converts itself into a 2/2 flier. The clever part is the self-checking trigger: the ability fires only while the permanent is still an enchantment, so it flips exactly once and then stays a creature forever, never reverting and never stacking. That one-time clause governs the whole bargain. You pay up front for a dead card, sit through however many turns it takes, and the body you eventually receive is modest and arrives entirely on the opponent's schedule, not yours, against decks that never cast a creature it simply remains inert. It belongs to an early-era family of cheap enchantments posing as creatures-in-waiting, dormant shells that a specific event activates. The narrow design idea (a sleeper that only matters when the matchup obliges) never had much pull, but the underlying mechanic of a permanent quietly graduating from one card type to another the instant a condition is met has been refined and reused many times since, usually with the payoff untethered from the opponent's cooperation.