C'Thun has always been one of my favorite cards, and I wanted to make a unique deck around him. So, this is what I came up with. I have been having a lot of fun with it in casual, IDK how well it does on the ladder. Do keep in mind this list is less competitive and more memey than other c'thun priests. I tried to put effort into an actual guide for this one, but sorry if it's not that great.

SoU Update: Added Activate the Obelisk and x2 Twilight Darkmender, and removed Zilliax, Shadow Word: Horror, and Forbidden Words. So far this has been working better, as the healing is much greater with this version, but further tester is required. I am also thinking of Octosari

Guide:

The idea is to try to build up your C'Thun with your various support cards like Beckoner of Evil, while also trying to shuffle your minions with Spirit of the Dead and drawing them with Bwonsamdi, the Dead for a lot of value. In an ideal world, you would be able to also keep playing down a wall of taunts for very cheap with the Spirit of the Deads on board. Then you play C'Thun for a lot of damage. I've had lots of games with this deck where C'Thun has had 30 attack or more. You want to try to control your opponent and survive until you win, so make smart trades, and use your Shadow Words and Shadowreaper Anduin to clear sticky boards.

General Mulligan: In most games you're looking for: Beckoner of Evil, Twilight Elder, Disciple of C'Thun. Against aggro you might want Shadow Word: Pain, Shadow Word: Horror, Disciple of C'Thun and any taunt minions (other than Twin Emperor Vek'lor). It's also acceptable in some cases to keep Madame Lazul, Spirit of the Dead, or C'Thun.

Card Choices:

 -Shadowreaper Anduin: Clears big scary minions, and hero power does a lot of damage, especially with minions created by Spirit of the Dead. NOTES: Once you play this card, you can no longer use your hero power for Hooded Acolyte, and your only healing option is Zilliax

-Zilliax: This card just fits into every deck, and it has a spot in this one too. It also synergizes with Hooded Acolyte

-Madame Lazul Nice all-around tech card, especially if she gets shuffled back by Spirit of the Dead

-Seance: Can copy our own minions (which in a lot of cases is worth doing), or copy a nice minion our opponent played for us

-Shadow Word: Horror: Anti-aggro tech

-Rabble Bouncer and Stonehill Defender: Nice taunts, also didn't have any Crazed Worshipper

Other cards to try:

Note: Stonehill Defender, Rabble Bouncer and Seance are flex cards, if you make replacements below I'd suggest taking those out first

-If you have Crazed Worshipper, I would put him in the spots of Stonehill Defender, Rabble Bouncer, or even Zilliax

-I don't have her, but I bet Barista Lynchen might work in place of  either a Seance or Stonehill Defender

-Twilight Darkmender: Heal yourself for 10 in a pinch

-Ancient Harbinger: Has a better chance of surviving with all the taunts, or can be shuffled back for another chance. Makes getting C'Thun easier, which might be good in games where Spirit of the Dead gets a lot of value. You can also use her to bait out resources or force your opponent to trade into her out of fear of you drawing C'Thun

-Doomcaller: Not sure about this one, might be good in niche situations where your C'Thun dies and doesn't get shuffled by Spirit of the Dead

-Emperor Thaurissan: Tick! Not sure how well this would work, but reducing the cost of all cards is always nice

If you have any suggestions, leave them in the comments, I'd love to hear them! If you enjoy the deck, an upvote would be nice.  :)

Edit: I swapped x2 Rabble Bouncer with x2 Crazed Worshipper, does indeed work better. I also swapped Stonehill Defender with Ancient Harbinger, but I haven't been able to successfully utilize it, so IDK if it's that good.

Edit 2: I am attempting to make C'Thun decks for every class, playing with Warrior and Mage next.