Bazaar of Wonders
A "World" enchantment was Magic's earliest attempt at a uniqueness rule baked into the type line: only one could exist at a time, and casting a second put the older one into the graveyard while the newcomer stayed in play. Most of the World cycle policed lands and global effects. This one polices names. The graveyard-exile clause on entry is housekeeping, not a trap: it wipes the slate so players are not instantly locked out of casting the spells already sitting in their yards when the enchantment resolves. From there, the lock does the work. Casting anything whose name already sits in a graveyard or on the battlefield as a nontoken permanent gets countered outright. The design logic is symmetrical and self-referential to a fault: it punishes redundancy, so the decks most hurt are the ones built on multiples of the same card, while singleton-heavy lists slip under it. The card you already played is enough to brick its twin. This is a designer's thought experiment given full form: what would a name-based stax piece even look like, and the answer is a piece that asks every deck in the room to stop running four-ofs. The friction is enormous, the symmetry mostly hurts whoever cast it, and the World supertype means it cannot even stack with itself. A genuine curio: an idea explored once, in full, and never quite revisited in the same shape.
