Karn, Silver Golem

The animation engine that turned every artifact in your deck into a threat, and the card that made a generation of players read mana value as a power-and-toughness stat. The activated ability carries the design: pay one mana and any noncreature artifact stands up as a creature sized to its cost, so a humble five-mana rock swings as a 5/5 while a zero-cost permanent becomes a 0/0 that quietly dies. That last detail is its meanest trick: animating an opponent's artifact lands turns them into 0/0s that fall off as a state-based action, a backdoor land-destruction line aimed across the table. In an artifact-saturated environment this opened attacks that did not otherwise exist, alpha strikes assembled from mana rocks and equipment, lethal damage hidden inside permanents nobody thought to block. The combat clause is what balances the body: the -4/+4 fires only when this golem blocks or becomes blocked, so it happily attacks into an open board as a clean 4/4, then collapses into a 0/8 wall the instant combat is contested, unable to trade in the red zone. That conditional inversion (a beater that turns into a fog the moment anyone engages it) reads as a puzzle rather than a brawler. It also seeded a name Wizards has returned to for decades: this is the original Karn, the artifact mind later reprised across multiple planeswalker incarnations, all of them carrying the fascination with the machinery of permanents that this activated ability first expressed.