Opal Caryatid

The whole design conceit is punishment disguised as a do-nothing: it costs one white mana up front and then sits inert until an opponent develops a creature spell, at which point it stands up as a 2/2 Soldier ready to block. That makes it a reactive defender paid for in advance and triggered entirely by the opponent's development, arriving without spending mana on the turn it matters and sidestepping the tempo cost of casting a blocker into an incoming attack. The catch is that you never hold the trigger. The animation waits on the opposing creature spell, so against control or a noncreature shell the card may simply never wake up, contributing nothing for the duration of the game. The clause checking whether it is still an enchantment is the tidy templating that prevents it from re-triggering once it has already become a creature. It comes from a vein of cheap early-era white permanents built on the same wager: pay almost nothing now, collect a body later, and trust the metagame to supply the creatures that flip the switch. The result is sharp against aggression and an embarrassment against anyone who declines to play creatures at all, a card whose entire value is mortgaged to an opponent's choices.