Wormfang Behemoth

The drawback is the engine. Emptying your hand on a 5/5 body reads as pure downside, but the second trigger is what the Wormfang cycle was built to abuse: any way to bounce or sacrifice the creature returns every exiled card to your hand at once. The behemoth is a storage device disguised as a tempo loss, and the cleanest combos pair it with a recursion outlet that resets the exile zone on demand. There is a subtler trick too: cast it as your last act before passing, exile a hand you would have discarded to your maximum-hand-size cleanup anyway, and you have effectively stashed cards past the discard step. This member of the cycle gets the largest stats, which is why it is the one that ever found a home in dedicated bounce shells rather than rotting in a binder. Read straight, it punishes you for casting it; read as a deck piece, the exile is storage, and the leaves-the-battlefield clause is a withdrawal slip. The whole Nightmare design hinges on whether you control the timing of that withdrawal, which is the difference between a 5/5 that costs you your turn and a 5/5 that hands it back with interest.