I hope you are enjoying the new season!  Here is a version of Mecha'thun Warlock that I've been trying out.  PLEASE CRAFT CARDS AT YOUR OWN RISK!!!

This is supposed to play like the old Mecha'thun deck, except you have the quest (Supreme Archaeology) instead of Emperor Thaurissan.  The combo pieces are still Mecha'thun and Cataclysm with Bloodbloom being optional this time.  If you reduce any of these 3 cards to 0 mana, you can win the game after you burn your deck! Adding Bloodbloom gives you a 3rd chance for a 0 mana card.  You could add a 2nd Cataclysm, for a 4th chance.  

To do this, I had to remove a few cards.  This was tough.  With the old list, The Soularium was only useful in a few matches.  Most of the time it was a dead card.  Cutting a Mortal Coil may not be the right move, but I didn't want to remove anything else yet.  I'm not sure if you need Hemet, Jungle Hunter or Mal'Ganis, but they're great cards....especially for 0 mana.

Some Card Explanations:

Sacrificial Pact - This deck needs this.  Usually use it on a Voidcaller, but that's not always the right move.

Mistress of Mixtures - Should be obvious.  Great for healing early and competing with aggro.

Mortal Coil - Great for card draw and setting up a Defile or another AoE.

Supreme Archaeology - The new Emperor Thaurissan for this deck.  Draw 20 cards and it gives you the Tome of Origination.  0 mana expensive cards can win games.

Bloodbloom - The 3rd option to be reduced to 0 mana by drawing it with the Tome of Origination.  If you use Bloodbloom to win, you will need a little health to off-set its battlecry.

Darkbomb - Great to get rid of that pesky 3-health minion on turn 2.  Don't always use this on the first minion you see.  I  try to save it for something that can hurt you like Underbelly Angler.

Plot Twist - Complete the quest quicker with this card.  Usually only use the 1st one early.  Against slow decks you might want to save it for another chance to shuffle in your combo if all three combo cards are in your hand (and none cost 0 mana).

Sense Demons - Great for cycling your deck.  Plus it gives you your demons - Voidcallers, Voidlords, and Mal'Ganis!

Cataclysm - This is the 2nd option card to be reduced to 0 by drawing it with you Tome of Origination.  You don't need Bloodbloom at all if you reduce Cataclysm or Mecha'thun to 0 mana.  You could run a 2nd one for a 4th option card to reduce to 0 mana.

Hellfire - I def think one is good.  Two may be better but IDK.

Voidcaller - Play this to cheat out Voidlord[Hearthstone Card (CardID or Name) Not Found]s and Mal'Ganis.  Make those mech hunters concede!

Hemet, Jungle Hunter - Similar to The Soularium, you don't want to play this card unless the situation is right.  You (usually) don't want to burn Plot Twist, or Bloodbloom which does happen once you play Hemet, Jungle Hunter.   The reason he's in there is if you have the combo ready to go and you need to burn your deck.  

Also, If you only have Mecha'thun (or Cataclysm) in your deck with 3 mana or less cards, you can play Hemet, Jungle Hunter then hero power to draw Mecha'thun (or the Cataclysm) for 0 mana.

Mecha'thun - The VIP.  1st option to be reduced to 0 mana.  Between this and Cataclysm it really doesn't matter.  Bloodbloom can be a little trickier if you're low on health.

Card Changes:

I'm not sure how many Mortal Coils to run.  Hemet, Jungle Hunter and Mal'Ganis can prob be removed...maybe for that second Mortal Coil.  I'm trying the legendary cards for now.  I'm thinking Mal'Ganis better than Hemet, Jungle Hunter.

Or, as I mentioned before you can add a second Cataclysm for a 4th card to reduce to 0 mana.   That may be better.  I'll def try this out.  It could make Reno Jackson worse though.

How to play around Mage secrets:

In general, I try to complete my quest as soon as possible so I can get some 0 mana cards in my hand.  Sometimes that's all you need to win, if you can get there and not draw poorly.  Against mage you have to worry about Counterspell and Potion of Polymorph.  Just save one extra 0 mana spell and one extra 0 mana minion from your Tome of Origination besides the 0 mana combo piece.  Wait till your deck is empty, then play the extra 0 cost spell and minion first, then play Mecha'thun, (Bloodbloom if nec) and Cataclysm.

I hope this makes sense.  Let me know if you have any questions or suggestions.  I'll update this soon.

UPDATE - 8/9/19

- Hemet, Jungle Hunter, + Cataclysm

Hemet, Jungle Hunter seems useless most of the time.  So I put in the 4th chance card to reduce to 0 mana: a second Cataclysm.    Impbalming is an interesting option to put more cards in your deck if nec.

Once you complete the quest, you only want to draw cards using your hero power until you get a 0 mana combo piece.

UPDATE - 8/13/19

- Cataclysm, - Mal'Ganis, + Hemet, Jungle Hunter, + Emperor Thaurissan

I mentioned above that this deck needs another win con.  So, I'm making the deck more like the old Mecha'thun deck with Supreme Archaeology and Plot Twist being used to help out.

So, the idea is if you complete your quest and you happen to draw a 0-mana Mecha'thun, Cataclysm, or Bloodbloom, you can win the game that way....just empty your deck.

But first I would try to acquire Bloodbloom, play Hemet, Jungle Hunter and win the "old" way.  This involves playing Emperor Thaurissan after you have Bloodbloom and Mecha'thun in hand.  Plot Twist can help you draw the right cards if you don't have them.

Other Card Options:

You can try a second Bloodbloom instead of Mortal Coil - not sure if it's needed.  Twisting Nether might be better than Hellfire - not sure.  Otherwise, I think the deck is solid.  I'll try a few of these changes and update again soon.