Bonehoard Dracosaur
The tax you pay for the engine is time: this is a five-mana 5/5 flyer with first strike, a fine body that attacks and blocks well above the ground on its own, but the card advantage does not start until your next upkeep, which means the creature has to resolve and survive a full turn cycle before it earns anything. That delay is the whole balancing act. Leave it alive one extra upkeep and it exiles two cards you may play that turn, and each category backfills a resource you might have spent getting there: a land you hit turns into a 3/1 Dinosaur, a nonland turns into a Treasure. The split is the clever part. You are rewarded no matter what the top of your library hands you, so the trigger never whiffs; it only varies in what it gives. What keeps it from spiraling is that the exiled cards are use-it-or-lose-it, playable only that turn, so the engine asks for a deck built to spend: develop the board, keep mana open, cash the free cards rather than bank them. Sit still and the value evaporates. The result is a threat that is a clock, a mana source, and a refill folded into one card, and one that punishes slow answers, because every upkeep it survives is another two cards deep the gap runs.
