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Zoolock is a deck archetype which tries to flood the board early on, taking control of the game and keeping it. A common misconception is that zoolock works like a face deck - it is NOT. Zoolock players must understand how to correctly trade and keep control of the board, as well as decide when to face and when to trade.
3 drop: Dragonblight Cultist
Game plan and strategy:
Zoolock doesn't exactly fit into either aggro, midrange, or control. It's comparable to both midrange and aggro, and changes its style based on the deck it is playing against.
The current iteration of the deck uses invokers with the almighty Galakrond, the Wretched to finish off its opponents with style after flooding the board with imps. It also runs a "discard your highest cost card" package.
Against aggro: Zoolock is actually a control deck! Try to contest board and get value trades - such as using two 1/1 imps to bring down a Huffer. Avoid going face until you've firmly secured the board!
Against midrange: Zoolock tries to play aggro in this situation. You should be able to gain board easily, and since midrange often has limited control tools, you can often swarm out your opponents with quick combos. Trade out your opponent's minions when necessary and hit face when your opponent can't punish.
Against control: Zoolock can both dominate and get dominated. As usual, flood the board early on with cards like Voidwalker and Flame Imp. You have to take more care in this matchup, though. Is your opponent a priest? Watch out for Breath of the Infinite. Druid? Better play around those Swipes. Correct positioning and holding back on plays is the key to victory here.
Upgrade Cards: *will edit more when I have time
Kronx Dragonhoof: Kronx is an absolute necessity in any galakrond deck. It either draws you your most powerful card or follows up on your most powerful card - either way, it's a doozy. Make sure you know what his devastations are - always have a plan!
Sea Giant: In this deck, where you're looking to flood the board with small creatures, Sea Giant will often be a 0 mana 8/8. It's just that good.
Diseased Vulture: A solid body with an ability that is easily triggerable and adds to tempo is absolutely amazing in this deck. Combine with cards like Neferset Thrasher and Darkglare for maximum self-harm synergy.
Veiled Worshipper: It shouldn't be too difficult to invoke two times, so this will very often be a 4 mana 5/4 with draw 3 on it. Ridiculous.
Chaos Gazer: Not too shabby. 4/3 is an aggressive statline, and the ability can screw up combo decks or just force the opponent to play suboptimally.
Arch-Villain Rafaam: TOGETHER, WE SHALL RULE DALARAN!
On a more serious note, Rafaam is pretty good. He provides a lategame gameplan for if and when this deck runs out of steam, and is especially good when you consider that there's a fair chance that you highroll Princess Talanji. He's not in the best iteration of this deck at the moment (since you may end up getting rid of the galakrond in your deck) but if you have him, he's still a fine card for when you go lategame.
Cards you may want to replace:
Anything you want to ask about? Comment, and I'll try to respond ASAP, as well as maybe add it to here!
Edit: 8/12 Revamp of deck. Now includes lackey synergy!
Edit: 8/15 Deleted some text about Leeroy Jenkins that no longer applied to the deck.
Edit: 12/10 Updated for Descent of Dragons, adding Galakrond synergies!
Edit: 12/11 Changed archetype from zoolock to Galakrond warlock (it's both, but Galakrond is more specific). Also, I'm sad to say that Hearthpwn is not considering this a budget deck, even though its only legendary was given out for free. I'm going to ask them if this can be changed, but if it can't be, this deck will not be visible by going to the budget section of the deck searcher.
Edit: 12/12 After seeing the meta shift on D1 of the new expansion, Faceless Corruptor seems to be absolutely insane. Getting it to key off a minion on turn 5 gets you a ridiculous number of stats. Therefore, since it's a rare and very important card, I've added it to the deck.
Edit: 12/16 Replaced Mecharoo with Murmy, since Mecharoo is rotating out of standard in April and is therefore a slightly less efficient craft. Updated the "Cards to replace" section by removing Mecharoo and adding Doubling Imp, which is still better to take out of the deck than Murmy. Also updated texts about Arch-Villain Rafaam and Leeroy Jenkins, who turn out to be slightly less viable than last expansion.
Edit: 1/17 Added Kronx Dragonhoof to the upgrade section.
Edit: 2/10 Added Chaos Gazer to the upgrade section.
Edit: 3/30 Removed Leeroy Jenkins from the upgrade section, since it is moving to the hall of fame.
Edit: 4/8 New expansion! Removed rotated cards, added in discard package. Updated outdated text.