Quest for Ula's Temple
The fantasy is enormous and the math is brutal, which is exactly why this card is beloved by the players who build around it. Three upkeeps' worth of cooperative top-decks, each gated on whether the next card off your library happens to be a creature, buys you the right to drop a Kraken, Leviathan, Octopus, or Serpent straight onto the battlefield at end step, paying none of its cost. Those creature types live in the highest reaches of the curve: eight- and nine-drop sea monsters that almost never see honest play, so the payoff is game-ending in a way one-mana enchantments rarely are. The tension is that the counter mechanism wants a deck stuffed with creatures to maximize hits, while the payoff wants a hand holding one of those few enormous beasts, and reconciling those two pulls is the whole deckbuilding puzzle. The quest only inspects the top card, so library manipulation that stacks creatures upward converts a coin-flip into a clock, and because the revealed card stays on top, a busy upkeep can advance the quest and still hand you the card you wanted for that turn's draw. It is a build-around in the truest sense: inert as a loose include, lethal as a thesis statement. Few cards ask you to commit this much of a deck to a single enchantment, and fewer reward the commitment with a swing this lopsided when the counters finally land.
