Im really trying to make it happen but god, that’s impossible. I don’t blame the meta, i play the design itself. It would never see play. While shaman or hunter steps can be completed with 2 cheap spells, priest need the whole turn. I know the pay off is to win the game but if you just can’t pull it off, whats the point? I playing something never saw on this game: my own deck against the net decks. Again, i don’t blame the meta or because im losing (im 50/50 on more than 30 games), im still refining the deck but the thing is i never, i said never had the opportunity to win the game through quest, actually i never happened to complete the quest, which is ridiculous. After completion you have to play Xyrella and pray to draw the card lol. I don’t know, looks like they never actually tested the card.
In the barrrens meta, when control priest was meta, I think the quest would have made the deck tierS.
It’s definetly the meta if quest priest isn’t good: the effect is absurd and way easier to accomplish than, for example, a mechatun OTK.
You also don’t have to sacrifice cards in your deck to play combo pieces: the only combo piece is the quest reward and the shard; if you don’t lose them, you are good: so it’s also harder to counter.
Aggro is probably the natural counter of the deck (also mutanus); that’s it.
It works as intended: if they wanted people to just win the game the turn after, they would have made Xirella to add the shard to your hand.
The shard is made to win against other control decks: of course in an aggro/combo meta it doesn’t seem good.
I would find harder to believe that you won more games thanks to the shard than the opposite 
the quest need a full redesign…if they fix everything its going to be 1 of the major problem to pop if this one dont get redesign
so dont get attach to it(if we’re ever going to see a fix to all the broken stuff…this one is getting touched at some point)
If you’re not concerned with climbing, you may try this homebrew…
I’m constantly trying this and that in it, but it’s a fun deck to play. If you draw well, you can out-heal and block aggro, but like you said, it’s rough…
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Minion Quest Priest
1x (1) Seek Guidance
2x (2) Mailbox Dancer
2x (2) Pandaren Importer
2x (2) Wandmaker
2x (3) Entrapped Sorceress
1x (3) Fairground Fool
1x (3) Flightmaster Dungar
1x (3) Mindrender Illucia
2x (4) Cheesemonger
1x (4) Spice Bread Baker
1x (4) The Nameless One
1x (5) High Abbess Alura
1x (5) Overlord Runthak
1x (5) Shadow Hunter Vol’jin
1x (5) Taelan Fordring
1x (6) Claw Machine
2x (6) Lightshower Elemental
1x (7) Mutanus the Devourer
1x (7) Silas Darkmoon
1x (7) Skeletal Dragon
2x (8) Mo’arg Forgefiend
1x (8) Natalie Seline
1x (9) N’Zoth, God of the Deep
The purpose of the Questline is to give Control Priest a wincon that isn’t “Run the opponent out of cards” or “Generate lots of random value” (non-Priest players tend to not like facing that kind of deck). At the same time it also needs to be slow enough that it doesn’t exclude everything that isn’t hyper aggro from the meta.
As far as I’m concerned it accomplishes both of those things.
It is the meta.
In a controls meta where people play reno decks, millions of heal, druid gaining 10,000 armour with bees, warlock that summons rows of taunts that summons taunts upon death, resurrect priests who heal constantly and try to get games to go into fatigue, I will craft this questline in an instant!
Unfortunately, where the meta is still a bit faster, it’s not quite viable. All you have to do is wait. When people start to use control to counter aggro, you will get your chance to use it effectively.
Have you tried it out in Duels? I know C’thun is far more viable in early rounds of Duels since the deck is smaller. Make sure you try it in casual Duels, not the paid Duels mode. Paid Duels is difficult even with a good starting deck.
I was playing Heroic duels and noticed a couple of ppl running the questline so I decided to try it the next time I got offered priest. I managed to pull it off ONCE with Mind Tether and Shadow Word Void in round 3. Otherwise, it just got buried in the deck and I won through other conditions or never completed the quest. When I look at how easy the Shaman one is to complete, I do wonder about this one. But like someone else said, you can’t have easy win conditions when the final outcome is to instakill your opponent.