Always wanted to throw C'Thun, Mecha'thun, and Valdris Felgorge together in a quest warlock and finally arrived at a version I like. This is a Tier III deck overall, maybe bottom Tier II, but can be crazy with the right draws. I hit the Mecha'thun combo almost consistently when the opponent has 14 cards left in their deck.
Overall, just have a ton of fun playing this!
Combo is reduce Mecha'thun and Cataclysm to 8 and 2 mana cost, respectively, using Drakkari Enchanter and Emperor Thaurissan. Draw all of your cards from your deck and play Mecha'thun then Cataclysm. Very often, you'll get a 0-cost Mecha'thun or Cataclysm and then don't need to reduce their costs