For me the problem is that several of the other lines (warlock and Pali) consistently complete turn 4 for a 7/7 with a game altering effect. Try controlling a paladin that spams 1 cost stuff early (Pali has tons of generation), gets a 7/7 turn 5 that turns any silver hand recruits into 3/3 (and likely has two since the deck plays our 1 cost your next hero power costs 0 friend as part of the chain completion...) then spawns 2 3/3 minions per turn for two mana, has spell control like omy and tons of low cost cards that now constantly fill the board with upgraded 3/3 minions. Please tell me how a priest replies turn 6 and 7 (with any hope of staying on curve, not burning out of cards, and not needing a literally perfect hand to survive. Forget completing the questline, priest is happy to MAKE IT to turn 8 (especially because they lost a card on the quest line, and only got it back if they played on curve 2,3,4 which again requires either perfect draws, of has allowed the paladin to melt your face turn 5.
Dont even get me started on warlock. Darkglare warlocks can get their quest line completed turn 4. Which means a turn 5 play 7/7. that will allow the warlock to low cost burn you to death by latest turn 6 (turn 7 without coin - even reno priest may not have time to even play reno - and even if you do, the warlock is drawing cards by burning you, so you are still gonna melt, even back at 30). Darkglare makes most of these plays free (or 1 mana) and many of the early ones don't even put the warlock at risk for aggro (lose 5 life, gain 5 armor etc.). Literally (worst case warlock goes first): turn 1, questline, turn 2 hero power, turn 3 dark glare + 2/1 draw and lose 2 life + 1/1 hero power free + hero power. Quest line completes both heros now at 27. Play lose 5 life gain 5 armor, play 3/2 lose 3 life. Quest line 2 completed play now stands opponent at 24 warlock at 25 with 2 armor, warlock with a 2/1, 1/1, 1/1, 3/2 in play, one mana open, 3 cards in hand. Assuming those 3 cards aren't crazy lucky (if they are, the quest line finishes here), the warlock can easily finish with a draw and self burn turn 4 (even if dark glare dies). Now the turn 5 7/7 is happening. The warlock is above 15 (probably well above counting armor) and had tons of early minions so is exceedingly unlikely to have been overrun by aggro, and can, turn 6, draw and burn through their deck at the other players expense (with coin, I've actually seen warlocks play the 7/7 turn 4 and kill me turn 5).
Anyone who thinks a questline that requires 7 distinct plays totalling 35 mana with restrictions on the timing of the plays (you need to draw and cast your 2,3,4 costs BEFORE the 5,6 costs count, so if you don't get fansastic luck and get a hand perfectly on curve (or play a discover deck and get reasonably lucky) you will be foddering plays saving yourself that dont count for the quest. Even when the quest completes all you get is a 7/7 fof 5 with the chance to draw a block able win condition (mutanus eats the 7/7, tickatus kills the shard, omy, counterspell, illucia, the list goes on) that costs 10 mana....
The questline is hot trash in wild and at best middle of the road in standard.
Even if they made a card use original cost of a card in effect it would still be trash. That would be my suggestion for standard.
When the community says please give priest a win condition I don't think this is what they meant. That said, this doesn't help priest as much as people think it does.
While obviously curvestone could occur with a turn 10 kill, aggro has been rampant and not enough has been seen to show it going in a control direction. This card doesn't really help priest win the aggro matchup, it's obviously just in line in a combo matchup, and in theory this would win in a control matchup. However if the quest existed in a control matchup you'd save mutanus for the turn the quest ended to eat xyrella, or in warlocks case Tickitus could just burn the shard itself. While this could be good I don't see it being nearly the gamebreaker people think it is.
Honestly at best a 2 star with current kits, and i'd bet it gets nerfed if it is too strong anyway.
I mean priest... doesn't even really struggle against aggro that much. They have lots of efficient removal tools and healing.
But seriously: at what point are people going to start reading the quest rewards for each step along the way?
Drawing 2 cards just from playing a single 2, 3, 4, 5, and 6 cost cards is really good.
It's not just about the 10 mana instawin card.
when a game ends on turn 5 or 6 hell even 7 its just bad card.