Warlock does have Backfire, Free Admission, Raise Dead and of course the hero power, so it is not that hard to get a full hand. The main problem is that the pay-off isn't particularly good because of the 6 health and lack of keywords.
I found a use for Serena once: as a decoy in the "Battle of the Bans" Tavern Brawl. I guess people didn't trust it and banned it out of an abundance of caution, while I just put it in there because I needed a backup minion.
It says a lot though that the card was most useful when it didn't see play.
While this would be better than Truesilver on curve if it had 4 attack, in the late game I think it would be a significant drawback that it cannot go face, both because it cannot be used as a finisher and because you might not be able to use it to heal at all if the opponent either has no minions or only high attack minions.
It's also a 4/2 weapon; it doesn't need much upside to be good enough. Maybe copying the deathrattle of Sneaky Delinquent or Devouring Ectoplasm is already enough in an aggressive Rogue deck.
Maybe it would have been playable if it always decreased one card and increased another card every turn. Then you would be sure to get one card to play ahead of curve and you could play the other card on a later turn when mana is less tight. Even with that change, I think it would need to be a 3/4.
I think it's more likely going to hinder than help you as a Quest Priest, since this undoes the curve planning you did in the mulligan and using discover effects. Maybe you could argue that you just don't play it if your hand already curves well, but then you need another 3-drop to play on turn 3.
And even if the effect would be a small net-positive, there is still the question of whether it's enough to compensate for the poor stats. For example Venomous Scorpid is more reliable in fixing your hand and provides a better minion on board.
At 3 overloaded mana crystals, a Lightning Bolt that unlocks them instead of adding one is already pretty insane. Even when you're overloaded for 2 it's probably still good, since it's still like having Arcane Shot and Innervate in one. And if you don't have any overload cards in your hand, you can just trade this in.
It's somewhat similar to Troublemaker. Overall, I think Troublemaker is probably more reliable (more stats, two attacks), but you could curve Lothar into Troublemaker.
Thinking about it more, Wandmaker and Venomous Scorpid are likely the reasons we're not seeing Annoy-o-Tron and Stoneskin Basilisk in decks: slightly worse minions, but getting a spell more than compensates for it.
Corpsetaker saw a lot more play after Zilliax came out. It would whiff if you drew Zilliax first, but if you drew Zilliax you'd probably be in a good position already.
I think there are plenty of good rush and taunt options in Standard at the moment. For divine shield, there are Annoy-o-Tron and Stoneskin Basilisk, which don't see play right now but were good cards in their original meta; I don't know if they were power creeped out or whether people just haven't given them a go yet as returning cards are less exciting than new cards.
In any case, I think that 7/7 rush, taunt, draw 2 is still a good deal for 8 mana: getting divine shield is a bonus rather than a necessity.
If you get multiple 5/5 rats in hand, you don't need to play them all out on the same turn if you expect AoE. Instead, you do your normal plays and add in one 5/5 rat each turn for a few turns in a row; at some point the opponent is likely to run out of answers.
Priest didn't play a win condition because it didn't need it. There were enough win conditions available in N'Zoth, Alexstrasza and Ysera, but they weren't included in top decks because focusing on just surviving is good enough and more consistent.
In many games, I think you'll miss a step in your curve at some point. From there on, you'll either have to play behind the curve or postpone quest completion. If you go second, you'll have a better chance to stay on curve due to starting with one more card, but you can't really get value out of the coin, so you're still behind your opponent's curve.
The quest itself eating up a card slot in your hand was a significant drawback of the Un'goro quests. Questing Explorer compensated for that in the Uldum era, but that card is no longer in Standard. You get to discover a card after the first stage, so you are compensated eventually for playing the quest, but you might have missed a curve beat by then already.
I think this would work best with mostly minions in your deck: they tend to be less situational than spells, so easier to play on curve. Having multiple minions in hand when you get Xyrella, the Sanctified will be important to dodge Mutanus the Devourer. Assuming you went first, played on curve and played no 0-cost cards, you should have 6 cards in hand after quest completion; if that is 5 minions you should be relatively safe, but if it is 2 minions you might see your quest line end prematurely. Also tutoring the final reward spell is more reliable if you don't have many spells in the deck.
All in all, I don't think this will be a top deck (which is probably for the better). Priest can already play minions on curve and end with N'Zoth, God of the Deep, which is more forgiving when you can't curve out perfectly.
I think they didn't use the legendary quest slot because it might be taken up by other events. For example, people that didn't finish their Fire Festival questline yet will still have that in their legendary quest slot.
As for the reward, it's still 50% more XP for a quest that is relatively easy to complete. And while Hearthstone is expensive, I don't think a monthly event would be the way to fix that. It's much easier to increase the gold to XP ratio, if they wanted to make the economy more budget friendly.
Warlock does have Backfire, Free Admission, Raise Dead and of course the hero power, so it is not that hard to get a full hand. The main problem is that the pay-off isn't particularly good because of the 6 health and lack of keywords.
I found a use for Serena once: as a decoy in the "Battle of the Bans" Tavern Brawl. I guess people didn't trust it and banned it out of an abundance of caution, while I just put it in there because I needed a backup minion.
It says a lot though that the card was most useful when it didn't see play.
While this would be better than Truesilver on curve if it had 4 attack, in the late game I think it would be a significant drawback that it cannot go face, both because it cannot be used as a finisher and because you might not be able to use it to heal at all if the opponent either has no minions or only high attack minions.
Or, given that it can't go face, would 4 attack be OP?
It's also a 4/2 weapon; it doesn't need much upside to be good enough. Maybe copying the deathrattle of Sneaky Delinquent or Devouring Ectoplasm is already enough in an aggressive Rogue deck.
But would that avoid your death? I know that lifesteal is applied before the player death check, but I don't know about weapon deathrattles.
I guess it would have to be: if you copied the deathrattle from Deranged Doctor, you'd expect it to heal you before you die.
Maybe it would have been playable if it always decreased one card and increased another card every turn. Then you would be sure to get one card to play ahead of curve and you could play the other card on a later turn when mana is less tight. Even with that change, I think it would need to be a 3/4.
I think it's more likely going to hinder than help you as a Quest Priest, since this undoes the curve planning you did in the mulligan and using discover effects. Maybe you could argue that you just don't play it if your hand already curves well, but then you need another 3-drop to play on turn 3.
And even if the effect would be a small net-positive, there is still the question of whether it's enough to compensate for the poor stats. For example Venomous Scorpid is more reliable in fixing your hand and provides a better minion on board.
Summon two 7-health minions... This card has potential!
My brain as I was reading the card text: "Why is this a Shaman card? ... Oh no."
Well, at least we're getting Rustrot Viper.
At 3 overloaded mana crystals, a Lightning Bolt that unlocks them instead of adding one is already pretty insane. Even when you're overloaded for 2 it's probably still good, since it's still like having Arcane Shot and Innervate in one. And if you don't have any overload cards in your hand, you can just trade this in.
It's somewhat similar to Troublemaker. Overall, I think Troublemaker is probably more reliable (more stats, two attacks), but you could curve Lothar into Troublemaker.
Thinking about it more, Wandmaker and Venomous Scorpid are likely the reasons we're not seeing Annoy-o-Tron and Stoneskin Basilisk in decks: slightly worse minions, but getting a spell more than compensates for it.
It is weird that they put the rarest keyword last. Maybe that was done as a kind of nerf after he was too good in playtesting?
Corpsetaker saw a lot more play after Zilliax came out. It would whiff if you drew Zilliax first, but if you drew Zilliax you'd probably be in a good position already.
I think there are plenty of good rush and taunt options in Standard at the moment. For divine shield, there are Annoy-o-Tron and Stoneskin Basilisk, which don't see play right now but were good cards in their original meta; I don't know if they were power creeped out or whether people just haven't given them a go yet as returning cards are less exciting than new cards.
In any case, I think that 7/7 rush, taunt, draw 2 is still a good deal for 8 mana: getting divine shield is a bonus rather than a necessity.
This might be viable in Standard, but even if it's not, I'm excited for Unseal the Vault in Wild.
If you get multiple 5/5 rats in hand, you don't need to play them all out on the same turn if you expect AoE. Instead, you do your normal plays and add in one 5/5 rat each turn for a few turns in a row; at some point the opponent is likely to run out of answers.
Priest didn't play a win condition because it didn't need it. There were enough win conditions available in N'Zoth, Alexstrasza and Ysera, but they weren't included in top decks because focusing on just surviving is good enough and more consistent.
In many games, I think you'll miss a step in your curve at some point. From there on, you'll either have to play behind the curve or postpone quest completion. If you go second, you'll have a better chance to stay on curve due to starting with one more card, but you can't really get value out of the coin, so you're still behind your opponent's curve.
The quest itself eating up a card slot in your hand was a significant drawback of the Un'goro quests. Questing Explorer compensated for that in the Uldum era, but that card is no longer in Standard. You get to discover a card after the first stage, so you are compensated eventually for playing the quest, but you might have missed a curve beat by then already.
I think this would work best with mostly minions in your deck: they tend to be less situational than spells, so easier to play on curve. Having multiple minions in hand when you get Xyrella, the Sanctified will be important to dodge Mutanus the Devourer. Assuming you went first, played on curve and played no 0-cost cards, you should have 6 cards in hand after quest completion; if that is 5 minions you should be relatively safe, but if it is 2 minions you might see your quest line end prematurely. Also tutoring the final reward spell is more reliable if you don't have many spells in the deck.
All in all, I don't think this will be a top deck (which is probably for the better). Priest can already play minions on curve and end with N'Zoth, God of the Deep, which is more forgiving when you can't curve out perfectly.
I think they didn't use the legendary quest slot because it might be taken up by other events. For example, people that didn't finish their Fire Festival questline yet will still have that in their legendary quest slot.
As for the reward, it's still 50% more XP for a quest that is relatively easy to complete. And while Hearthstone is expensive, I don't think a monthly event would be the way to fix that. It's much easier to increase the gold to XP ratio, if they wanted to make the economy more budget friendly.