This is cheaper than Arbor Up though, so it could be played on the turn before to trade without losing power on the board. If Token Druid is meta, this card is likely to be good in the mirror match at least.
What I'm more worried about is that when your opponent doesn't play many minions or only plays big minions, you won't get a lot of value out of this. The stats are such that you need to summon at least one Wyvern to make it worth the mana.
The card text says "all enemies", so that can be an advantage over Hellfire and there is no risk of damaging your Imp.
While the card seems useful, I do share the concern that 5 mana might be too much. It could be worth it if you want tutor a particular spell, but for the average deck there are likely better alternatives.
I'm sure it was limited to a fixed pool at some point in time, had something to do with the layout of the card text requiring manual work. Did they later implement auto-layout and remove the restriction?
The problem with teching against Rez Priest is that you'll face ten Odd Hunters and Pirate Warriors between each Rez Priest. It might still be worth it for the satisfaction, but not for the win rate.
You'll have to mana cheat it though, because if you play this versus an aggro deck on turn 7 you're probably either dead already or will die to the sheep, which would be most embarrassing.
As far as I know the beast pool for Deathstalker Rexxar is limited to the beasts that were in Standard when the death knights rotated. For other cards that generate random beast cards Explosive Trap would be useful though.
You could play a weapon on turn 4 and still have something to do on turn 5.
I think the value in the late game is actually fine: you are going to get a ~3 mana discount on a big demon and on the turn that the sigil resolves you have all your mana available, so you can put down multiple threats at once which your opponent might not be able to deal with.
When the battlecry resolves, Cera'thine is on the board, so she won't transform herself. Therefore it shouldn't matter exactly when the unmask happens, since you played a rogue card in any case.
Edit: Sorry, I didn't realize you were talking about what "other classes" means exactly. I thought you wanted to avoid unmasking.
Why is Maestra an obvious inclusion though? You'll get the Warlock hero power 1 out of 9 times, but other than that I don't see any real synergy.
One possible deck would be about half cheap spells and half 5+ mana minions and then run Vanndar Stormpike as well.
A different approach would be to run only 2-4 mana minions and Taelan Fordring. In particular Encumbered Pack Mule would be great in such a deck: play it early vs aggro, keep it in hand for a later tempo swing against slower decks.
Is it a Frost spell though? While there is plenty of snow in the artwork and there is "ice" in the name, the Frost tag doesn't appear in the video nor on playhearthstone.com.
Zoo was my first thought, but after some math I realized that you're going to get less than 1 token per turn on average, so I think the comeback potential from the tokens is rather limited. Drawing three cards is useful of course, but Backfire does that for less mana and leaves your board alive.
I think Jaraxxus is more powerful if you get into the position to play him, but Tamsin is easier to play because of the lower mana cost and the AoE from the battlecry makes it possible to play her when you are behind.
This is cheaper than Arbor Up though, so it could be played on the turn before to trade without losing power on the board. If Token Druid is meta, this card is likely to be good in the mirror match at least.
What I'm more worried about is that when your opponent doesn't play many minions or only plays big minions, you won't get a lot of value out of this. The stats are such that you need to summon at least one Wyvern to make it worth the mana.
There is no way to know for sure just from the card text, but I think it's more likely that only the base damage is boosted and the Fel bonus is not.
The card text says "all enemies", so that can be an advantage over Hellfire and there is no risk of damaging your Imp.
While the card seems useful, I do share the concern that 5 mana might be too much. It could be worth it if you want tutor a particular spell, but for the average deck there are likely better alternatives.
I assumed this turn from a balance perspective, but when taken literally the card text does suggest it is permanent.
I'm sure it was limited to a fixed pool at some point in time, had something to do with the layout of the card text requiring manual work. Did they later implement auto-layout and remove the restriction?
The problem with teching against Rez Priest is that you'll face ten Odd Hunters and Pirate Warriors between each Rez Priest. It might still be worth it for the satisfaction, but not for the win rate.
You'll have to mana cheat it though, because if you play this versus an aggro deck on turn 7 you're probably either dead already or will die to the sheep, which would be most embarrassing.
Possibly Zoolock too, with Backfire and Hullbreaker. Maybe go really fancy and throw in Altar of Fire and Neeru Fireblade as well.
As far as I know the beast pool for Deathstalker Rexxar is limited to the beasts that were in Standard when the death knights rotated. For other cards that generate random beast cards Explosive Trap would be useful though.
Since the extra community reveals are replacing the live stream, it's also possible they'll happen on the 24th.
You could play a weapon on turn 4 and still have something to do on turn 5.
I think the value in the late game is actually fine: you are going to get a ~3 mana discount on a big demon and on the turn that the sigil resolves you have all your mana available, so you can put down multiple threats at once which your opponent might not be able to deal with.
It's a sigil, not an objective, so it only lasts one turn.
When the battlecry resolves, Cera'thine is on the board, so she won't transform herself. Therefore it shouldn't matter exactly when the unmask happens, since you played a rogue card in any case.
Edit: Sorry, I didn't realize you were talking about what "other classes" means exactly. I thought you wanted to avoid unmasking.
Why is Maestra an obvious inclusion though? You'll get the Warlock hero power 1 out of 9 times, but other than that I don't see any real synergy.
One possible deck would be about half cheap spells and half 5+ mana minions and then run Vanndar Stormpike as well.
A different approach would be to run only 2-4 mana minions and Taelan Fordring. In particular Encumbered Pack Mule would be great in such a deck: play it early vs aggro, keep it in hand for a later tempo swing against slower decks.
Is it a Frost spell though? While there is plenty of snow in the artwork and there is "ice" in the name, the Frost tag doesn't appear in the video nor on playhearthstone.com.
Not bad, but it competes with Lightshower Elemental which is probably better unless you're up against a wide board.
This could be an on-curve follow-up to Nerubian Egg.
There is also synergy with Dreaded Mount, as Tamsin's Dreadsteed is 4 mana.
Tamsin's Phylactery is another new card with deathrattle synergy, but we probably need a few more for it to become a viable deck.
Zoo was my first thought, but after some math I realized that you're going to get less than 1 token per turn on average, so I think the comeback potential from the tokens is rather limited. Drawing three cards is useful of course, but Backfire does that for less mana and leaves your board alive.
I think Jaraxxus is more powerful if you get into the position to play him, but Tamsin is easier to play because of the lower mana cost and the AoE from the battlecry makes it possible to play her when you are behind.
Maybe both could even exist in the same deck?
If Diablo in Duels sticks around, Lord Barov could do the trick.