Hearthpwn

Full Combo (25 damage) - 6 cards:
Turn 9: Skull of the Man'ari (only if you have the combo cards in hand)
Turn 10: Man'ari pulls out Doomguard. Attack for 5. Carnivorous Cube the Doomguard. Prince Taldaram the Carnivorous Cube. Cast Dark Pact on both the Carnivorous Cube and the Prince to get 4 Doomguards. Attack for 20.

Partial Combo (15 damage) - 4 cards:
If you only have Carnivorous Cube and 1 Dark Pact, you can still do 15 damage by omitting the Prince and 2nd Dark Pact from Full Combo. Note: you can do this one earlier than full combo if you have already damaged your opponent enough.

Other card explanations:
Rest of the deck is either copies of the win condition (2nd Cube and 2nd Doomguard) or removal/survival.

Kobold Librarian - amazing draw on turn 1

Mistress of Mixtures - Early game survival, healing

Mortal Coil - card draw, small removal

Bloodmage Thalnos - card draw, spell damage for removal spells

Defile - board clear, even more effective with Tainted Zealot or Bloodmage Thalnos

Tainted Zealot - spell damage for removal

Prince Taldaram - combo card, still copies the deathrattle of Carnivorous Cube, so it'll summon two Doomguards as well

Hellfire - removal

Lesser Amethyst Spellstone - removal, healing, even better if you have in hand while Hellfire or Kobold Librarian

Carnivorous Cube - combo card, will summon back two Doomguard with charge as part of combo, only need one for combo so you can use other one if needed on Thalnos for card draw or Mistress for healing

Doomguard - combo card, main damage dealer. Only need one for combo, but play other one at your own risk, since you don't want to discard combo pieces. 

Skull of the Man'ari - combo card, must have. Must play this prior to your combo turn, so that at the start of your turn you get the free Doomguard.

Tar Lurker - survival, big taunt

Siphon Soul - large removal, healing

Twisting Nether/DOOM! - playing 2 of these, any combination works, large board clear to set up for combo, remove enemy board, card draw from DOOM! if needed.

Bloodreaver Gul'dan - if you don't kill your opponent with the max 25 damage combo, and they manage to clear your board of 5/7 demons, you can summon all of them back, with charge and attack face again!

Notes on the combo:
I know this combo is vulnerable to weapon removal, but I haven't seen a ton of that lately in this meta, so until that shifts, try out the combo. It's fun when you pull it off! You can also try to roll the dice and play a Doomguard early without the Skull, if you've got both Carnivorous Cube and Dark Pact in hand or something, with some other random removal cards. And then the following turn, if you didn't discard the pieces, go ahead and go nuts with the combo. You may have to try that anyways if Skull is at the bottom of your deck. Your hero power, combined with some of the card draw, should get you deep into your deck pretty early though, to cut down on those odds.

Mulligan:
Always keep Skull of the Man'ari. You don't want to risk it being on bottom of deck if you can keep in hand.
Keep: Any of your 1-2 cost cards, except Dark Pact, you can draw that later since it's combo. Mulligan anything else except Skull.

Hope you enjoy this deck! Give it a thumbs up if you like it. :)