do you remember echo giants using Echo of Medivh and Duplicate?, well this might increase that deck's power vback into slightly viable in wild, i'm not too sure how to evaluate this in standard though
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Big priest was never a problem until Lesser Diamond Spellstonegot printed, and this spellstone breaks way too many decks asides from big priest, basically compare it to gallery and you can see that spellstone should cost more than gallery, barnes isn't a problem in any other deck, spellstone tho, spellstone is a problem in many different priest decks both in standard and wild.
Even if you don't have a good opportunity to use this secret with a really powerful minion, it makes your opponent think before trying to trade against your Lich King and may force them to waste an attack on a less dangerous minion.
This fits really well into a midrange or control deck. I think that since all of the Mage cards have been pushing towards those types of decks, this will be really good. I think we'll spend a lot of time cursing this secret when we don't have the spell removal to completely deal with those large bodies.
Also a huge this is huge for playing Doomsayer against zoo. This card means they'll need to spend 14 damage to get through it, or waste most of their spell based removal
I disagree... I definitely think this will see play...
I'm not saying the card is bad nor am I saying it's not a cool effect, but would you really play it? If you are playing a control mage, you'll definately prioritize 30 other cards above this one, even if you are running secrets in a control deck, you are still playing Counterspell and Ice Barrier above this. In big spell mage this will be an inconsistensy. You might run it in big hand mage or odd mage, but I don't think it's likely. You might run a single copy in secret mage, even though Explosive Runes and Mirror Entity are way better in that archetype as they are much faster. I also think people overestimate how hard this is to play around, as Control Deck usually can just kill your big minion with removal and aggro decks will just ignore it.
TL:DR This card is cool but too inconsistent and all in all you probably have 30 other cards which are better for your mage archetype. The main problem is that you can't play it the same turn as your lich king. It's a good spell to discover though.