Worldgorger Dragon

The downside printed on this card is so catastrophic that it became the engine. Exiling everything else you control when it enters reads like a cruel joke until you pair it with an Animate Dead or Necromancy: the enchantment animates the Dragon, the enter trigger exiles the enchantment along with all your other permanents, which kills the Dragon (the enchantment is gone, so nothing keeps it on the battlefield), which fires the leave trigger and returns everything, including the enchantment, which animates the Dragon again. The loop spins infinitely, blinking every other permanent you control through exile and back. Each cycle re-triggers every enters-the-battlefield ability you have in play, so a single mana producer that taps for value or a card-draw permanent turns the loop into arbitrarily large mana or an empty library. The combo is fragile in a specific way: it is a true infinite loop with no built-in stop, so the deck has to interrupt it on its own terms (often by redirecting the returning reanimation aura to a different creature in a graveyard as the enchantment comes back) to actually win rather than draw the game out. The result is one of the most famous combo kills in the game's history, a piece built entirely around weaponizing a creature whose drawback should have made it unplayable, and the reason its name is shorthand for an entire reanimator loop rather than a 7/7 flier.