Arcum's Astrolabe

The trick that made snow relevant again. Before this, snow-matters mechanics were a curiosity from a single old plane, appreciated for flavor and ignored for power. This turned the snow supertype into a resource: a one-mana artifact that replaces itself, fixes every color, and quietly checks the box that a whole suite of snow-payoff spells wants checked. The genius and the problem are the same clause. Because it costs one snow mana rather than one generic, it demands a snow-basic manabase, but it then rewards that manabase by making every basic a rainbow source through its own activation. That closed loop (snow lands fund the Astrolabe, the Astrolabe fixes the snow lands) let three-, four-, and five-color decks run stripped-down manabases with almost no color risk, and the card-draw on entry meant the fixing came at no real cost in cards.

That combination proved too clean. Wizards banned Arcum's Astrolabe in Modern and Pauper in mid-2020, an unusually fast turnaround for a card that read like harmless fixing, precisely because it removed the deckbuilding tension that is supposed to price multicolor greed. The lesson it embodies is a durable one for artifact and mana design: a fixer that also cantrips and asks for a payment you were already going to make is not a fixer, it is a free spell that untethers a color pie. The snow requirement looked like a cost; it was the enabler.