#1. Defile
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Expansion: Knights of the Frozen Throne
Defile is a unique card with probably the best design ever (according to me at least). It is a powerful board clear if you have 1-2-3 etc. health minions which can give you a full clear. The card is known for being a "Defile puzzle" where players can figure out tricky board clears by getting the correct sequence. The final puzzle in the Boomsday Project's Puzzle mode is a Defile puzzle (and boy is it hard).
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Honourable Mentions
Clutchmother Zavas
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Expansion: Journey to Un'Goro
Clutchmother Zavas is the first type of Discard card to return to your hand after discard. Silverware Golem was the only other viable discard mechanic minion before this card. Zavas is leaving at an interesting period, where the discard mechanic has gotten better. The Quest Lakkari Sacrifice also goes with Zavas.
Cataclysm
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Expansion: Kobolds & Catacombs
Ah, the Deathwing spell. Cataclysm saw a lot of popularity where you could complete the Warlock Quest in just one turn. When Mecha'thun was released, it combined with Bloodbloom (using health to cast a spell instead of mana) to cast Cataclysm and destroy all minions, winning you the game.
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