Excalibur, Sword of Eden

A twelve-mana equipment reads like a joke until you look at what pays for it. The cost reduction scales off the total mana value of your historic permanents, and "historic" is one of the broadest brackets in the game: every artifact, every legendary, every Saga you control. Build a board that leans on those types (and a deck committed to legendary bodies will lean that way anyway), and the collapses toward something castable, sometimes toward nothing at all. This is the payoff-as-cost design pattern turned into a single card: the more you have already invested in the axis the card cares about, the cheaper the card that rewards that axis becomes. The buff itself is deliberately blunt, +10/+0 with vigilance, a number chosen to end games rather than nudge them, and the equip restriction to legendary creatures keeps the swing thematically fenced: this is the sword that only the worthy carry. What makes it interesting as a build-around is that the reduction and the equip clause point at the same requirement from two directions. You want a legendary to swing with, and stacking legendaries and artifacts is exactly what drops the casting cost, so the deck that can afford Excalibur is by construction the deck that can wield it. The printed number at the top is a threat and a lie at once: nobody pays twelve, and the card is built on the assumption that nobody will.