We need to be thinking about making cards work. A versatile, obviously good card would be OP and boring. It would edge out existing cards and allow for fewer choices in the game.
Does anyone know about the OTK shaman that popped up after GvG? That deck sits with a full hand pretty regularly and likes reactive removal like this (and of course BGH) and spells. That deck famously runs Malygos with Ancestor's Call. Since it sits with a full hand anyway, the deck should be happy with 2 Twilight Drakes and maybe 2 Azure Drakes. You've now got 5 sturdy dragons to trigger Rend, helping you survive until that nasty combo is ready.
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Quick and dirty feedback:
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Pit Lord is a warlock card. If priests could run Pit Lord, you'd better believe they would be all over that shit.
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Trying to subtract the stats of the average minion it summons from the dragon's stats is a bit reductive. It's actually better than that makes it seem, because you get the first choice of how to trade against the opponent's new 1-drop. If you have control of the board or if the board is even, you can probably kill the 1-drop for "free" (hit it with a Harvest Golem or Spider Tank or whatever you played on turn 3, hit it with a weapon, run some tokens into it, etc.).
Hungry Dragon is really good.
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There's some nice late-game combo action with this guy and Cabal Shadow Priest, MC Tech, etc., and it does play well with the new Blackwing Technician, but I think people are overlooking how strong this card is in a basic zoo-type deck. Playing it on curve following a 1-drop, 2-drop, and 3-drop? That's brutal. You've probably got something on the board that can kill the 1-drop it summons without dying, and what can your opponent do? Even if he's got removal for the Hungry Dragon, he basically concedes tempo to you permanently.
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Bad stats are irrelevant if the card has a substantial impact on the board (Al'Akir has terrible stats for 8 mana, for example). What's going to kill an 8/4? The a big-ish minion or the same removal that will kill any 7-drop other than Dr. Balanced (Poly, Hex, Fireball, lucky Crackle, etc.). If you hit an opponent's legendary with Rend, you got solid two-for-one value. Guaranteed. (Okay, if your opponent throws down Flamestrike against a board that he was going to Flamestrike anyway and gets Rend in the bargain, sucks to be you—but you probably shouldn't be playing Rend in that super-specific situation.)
It's true that Rend will be pretty weak against two of the most widely played legendaries, Dr. Boom and Sylvanas, and no better than a BGH against stuff like Rag and Mal'ganis. However, he'll be great against Loatheb (played just as much as Boom/Sylvanas, if not more) and against lots of other popular non-BGH-able cards: Harrison Jones, Tirion, Ysera, Antonidas, KT, Black Knight, etc. etc. Extra special lore/gameplay synergy: play Rend with The Beast, use him to zap Finkle Einhorn (kidding).
Basically, I think Rend will be playable in any dragon-based deck. That's his real weakness, the requirement that you hold a dragon. Even in a deck with lots of dragons, that's going to be a nuisance—you've got Rend and one dragon in your hand. Are you really going to hold that dragon in case the opponent plays Loatheb or something? Still, I think he goes in most dragon-themed decks.
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I think Acolyte of Pain is a very useful reference point for this card. Weak for the mana cost, with a relatively squishy 3-health body, but lots of upside if you can get it to take damage a couple times without dying.
In theory, you either buff its health so it can take extra hits, or you proactively damage it yourself. In practice, the first tactic just doesn't work: if you play Acolyte of Pain with Velen's Chosen/Hobgoblin/Mark of the Wild/Kings/whatever, it just draws removal. The only classes that ever play it in constructed, and the classes that use it best in arena, are mage and warrior. And I think it's pretty obvious that Grim Patron won't work with mage (although somebody will make a hilarious gimmick deck with Pyros and Echo of Medivh).
I think it might work in warrior. You've got your crazy Grim Patron + Warsong Commander + Commanding Shout board clear to dream about, but it actually just fits pretty smoothly into a deck that's already running Unstable Ghoul, Armorsmith, Acolyte, Whirlwind, Death's Bite, etc. Unlike mage, warrior can actually activate Grim Patron on turn 5 consistently. Maybe it goes in a slightly more aggressive take on your bog-standard control warrior, perhaps with a Bouncing Blade thrown in for funsies. Won't be super-competitive though.
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The 9-drops both look like Nefarian—once in dragon form, once in human form. Maybe he transforms? Maybe there's a different version of him depending on which class your opponent is playing, that'd be cool.
There are five of these at the beginning of the trailer, starting with Rend (which we know is legit). The second one is some kind of blue-ish drake breathing fire, don't know who that would be. The third is, a friend tells me, a boss from Blackrock Descent (I played WoW back in the old days, didn't make it as far as Cataclysm). Even though Rend is definitely a real card and Nefarian almost undoubtedly will be, I don't think these are all collectible. There are probably only five legendaries in the adventure (17 cards from bosses, 9 from class challenges, one legendary from each wing); one is presumably Emperor Thaurissan, and another will be something from MC. Then there's Rend and most likely Nefarian, which leaves only one other—probably something from Blackrock Descent.
Anyway, have some screenshots.