Mystic Enforcer

Threshold's poster child, and the card that taught a generation what the mechanic was supposed to feel like. Before the graveyard fills, it is a 3/3 with protection from black: a defensive body that walks through the dominant removal color of its era and trades up against black creatures with impunity. Cross threshold and it snaps into a 6/6 flier, a clock that closes games in four swings and refuses to be blocked on the ground. The genius is the gradient. Most threshold designs do nothing, or close to nothing, until the count is met; this one is already a real card from the turn it lands and a bomb once the yard is deep, so the deck never draws it dead. The requirement also nudges the builder toward exactly the cheap-spell, self-milling shell that wants a green-white protection beater anchoring it, which is why it slotted so cleanly into the threshold midrange decks that defined the archetype. Protection from the premier removal color of its day, plus evasion, plus a body too large to chump cleanly, is the trifecta that lets a four-mana creature actually end games. And tying the upgrade to a self-built resource rather than a mana payment is what kept it from being simply overpowered: you buy the back half with deckbuilding discipline, not with the cards in your hand.