Hearthpwn

This deck was inspired by the old demon-lock decks, that included cards such as Baron Rivendare and Kel'thuzad (which are also optional substitutions for cards in this deck.) The goal is simple - try and play your ancient watchers or Nerubian Eggs on turn 2, and follow it up on curve with a turn 3 Void Terror or Imp Gang Boss. Voidcallers are excellent in this deck, since on average they will bring out the bare minimum of an Imp Gang Boss or Void Terror (although try to avoid this) and they have the potential to summon Mal'ganis or Lord Jaraxxus; one of the more combo-ey parts to this deck. The idea is to control the board with huge minions, and protecting them with cards like Sludge Belcher, whilst getting in as much cheeky face damage as possible, (although prioritise trading efficiently in order to control the board). If things get out of hand, you can sacrifice a high attack minion with Shadowflame, in order to destroy your opponent's minions effectively. 

If you need to make substitutions, Barnes fits quite well into this deck, perhaps in place of Loatheb or one of the more expensive legendary minions (since this isn't a particularly cheap deck to make), but you can also add in Baron Rivendare or Dreadsteeds, since they combo reasonably well with Void Terrors anyway. You can be a little flexible, and try out different versions of the deck, maybe some with Barnes, or with/out Kel'thuzad and Dreadsteeds - perhaps even strong Deathrattle cards like Piloted Shredder would work out excellently in this deck. (Maybe even Feugen and Stalagg? Who knows?)

General mulligan strategy is to keep Ancient Watchers and Nerubian Eggs, as well as other strong early game cards like Mortal Coil and Imp Gang Boss. Against non-aggro matchups, it's generally more than OK to Life Tap on turn 2 and sometimes even turn 3, since the strength of the combos in this deck more than make up for it. Against aggro and zoo decks, it's a wise idea to try and mulligan for Shadowflame, Power Overwhelming, and Mortal Coil, but in a pinch, the one copy of Sacrificial Pact can sometimes make up for it, since usually you'll find yourself with a couple of stray 1/1 imps anyway.

That about concludes it, just remember the core principles of this deck, which are to mulligan for early game, but tapping is generally an OK option too. Use Void Terrors effectively, you only get a maximum of two shots at it per game, and it's a wise idea to not play them on turn 3, unless you have a Nerubian Egg or Ancient Watcher in play. Defender of Argus combos excellently with those cards too. Control the Board, and with cards like Dr. Boom and Imp Gang Boss, you should fill up your board pretty quickly. Against control oriented decks that somehow survived the Voidcaller into Mal'ganis combo, 2x9/7 Void Terrors and numerous legendaries, you have the power houses of Lord Jaraxxus and N'zoth, the Corruptor for extra mmph in the late-game.

Thank you for reading and be sure to give this deck a try, I play it often at ranks 10-20 and it seems to be very effective against a lot of decks. It's also a ton of fun to play and it feels great when your opponent plays Jaraxxus and you've still got your Sacrificial Pact in hand :)