Hogaak, Arisen Necropolis

The line "You can't spend mana to cast this spell" is the whole architecture: this is a creature that refuses the currency every other spell uses, and demands you assemble it from bodies and buried cards instead. Convoke taps your board, delve eats your graveyard, and together they turn a 7-mana 8/8 into a first-turns-of-the-game threat for a deck already flooding both zones. The graveyard-recast clause is what closed the loop and broke the leash: sacrifice it, dredge or mill it back, and reassemble it for free again, so it functioned less as a card than as a permanent that could be re-summoned out of a self-filling engine. That engine proved uncontainable. The card warped an entire graveyard-aggro archetype into the dominant deck of its format almost immediately, drew emergency errata to its supporting cast, and was ultimately banned in the format it was built for after a rare mid-cycle intervention, all inside a single summer. What made it so hard to answer was the same thing that made it fun to build: every restriction attached (no mana, exile your graveyard, tap your creatures) was a resource the deck was generating anyway, so the costs read as speed bumps rather than brakes. It stands as a cautionary case study in alternative-cost design: when the payment mechanism runs on resources the archetype already overproduces, the cost is not a cost at all.