Planar Gate

The cost-reduction-on-a-permanent template starts here. Before this, ramp meant mana rocks and lands; getting creatures down cheaply meant one-shot rituals or narrow tricks, not a standing discount that hung around between turns. Planar Gate proposed something different: a static, color-agnostic reduction that applies to every creature spell you cast while it's on the battlefield, sitting in the artifact slot rather than asking for a tribal or color commitment. The six-mana price tag is what keeps the design honest. You pay it up front, you survive the turn you tap out, and only then does the engine begin paying you back two mana at a time. That tempo trough is severe enough that the card never found a serious home, but the shape it introduced (a permanent that quietly reduces a whole category of spells) shows up again in Helm of Awakening, Semblance Anvil, Goblin Warchief, Urza's Incubator, Herald's Horn, and eventually the cost-reduction commanders that anchor entire archetypes. Most of those successors learned the lesson Planar Gate taught by being too expensive: cut the upfront cost, narrow the discount, or attach the reduction to a body that justifies the investment on its own. The original is a museum piece, but the museum is full of cards that descend from it.