This is a somewhat standard Demon Zoolock that uses Dreadsteed for explosive combos.
Reasons why Dreadsteed is amazing:
1. It has infinite* value.
2. It pretty much reads "4 mana: deal one damage each turn for the rest of the game or until silenced."
3. It combos incredibly with other cards in the deck.
4. It allows you to always* have a board presence turn 4+.
1. The most effective combo in this deck is Dreadsteed + Baron Rivendare which allows you to multiply your Dreadsteeds for even greater value (NOTE: You do NOT want to fill your board with Dreadsteeds, you want 4-5 AT MOST.)
2. Adding onto the Dreadsteed + Baron Rivendare combo you can also add Mortal Coil or Sacrificial Pact to allow for instantaneous multiplication of your Dreadsteeds and also get the added benefit of card draw or health gain depending on which card is used.
3. Dreadsteeds + Power Overwhelming is an incredible combo that not only allows for huge trading/burst potential, but also negates the drawback of Power Overwhelming, allowing you to use it without sacrificing any minions!
4. Dreadsteeds + Void Terror = free Void Terror Buffs without losing board control (Throw a Power Overwhelming on the Dreadsteed and trade/go face first for maximum effectiveness!)
5. Dreadsteeds + Mal'Ganis is a huge late-game combo that allows for near-infinite trading without losing board control, as long as you can keep Mal'Ganis alive you can't lose.
6. Baron Rivendare + Voidcaller is a huge tempo swing in your favor, try to get a Dreadsteed or out for free and combo it with Baron Rivendare on the same turn.
7. Dreadsteed + Knife Juggler is amazing for removal of small enemy minions.
Rundown of card choices:
Sacrificial Pact: Usually a bad card because it sacrifices board control for health gain, but with minions like Dreadsteed and Imps from Imp Gang Boss and Imp-losion there are plenty of targets for quasi-free health gain, and it combos extremely well with Dreadsteed + Baron Rivendare.
Mortal Coil: Great for removal/combo with Dreadsteed and gives card draw.
Power Overwhelming: Allows for trading up or reach for a final push for lethal, also combos well with Dreadsteeds.
Flame Imp: Staple Zoo/Demonlock card for early board advantage.
Voidwalker: Another good early game card to gain board advantage, look to buff it with Demonfire for added protection.
Demonfire: Good card due to all the Demon synergy, also acts as decent early removal.
Ironbeak Owl: Every deck needs 1 silence.
Knife Juggler: Combos well with Imp Gang Boss, Imp-losion, Dreadsteed and Nerubian Egg.
Nerubian Egg: Great synergy with Power Overwhelming and Void Terror, also works well with Baron Rivendare.
Imp Gang Boss: Great synergy with the other cards in the deck, staple in any zoo/demon deck.
Void Terror: Free buffs from Dreadsteeds/Imps. Buff them with Power Overwhelming beforehand for an even bigger boost.
Imp-losion: Great Demon synergy, amazing early removal. Combo with Knife Juggler early game, or Mal'Ganis late game.
Baron Rivendare: Mostly seen as a "fun" card, this has amazing synergy with the deck and enables near infinite value with Dreadsteeds.
Dreadsteed: The core card for this deck, it has near limitless value when played correctly and can single-handedly control the board. Irreplaceable for this deck.
Voidcaller: Staple in any Demon deck. Enables huge tempo swings on its own, even bigger ones when combo'd with Baron Rivendare.
Doomguard: Staple in most Warlock decks due to its amazing stats and charge. Great synergy with the deck, allows for efficient trading or reach as a finisher. (Try to get it out of Voidcaller for even more value)
Anima Golem: Great synergy with Dreadsteed, has amazing stats and can bait out removal making Mal'Ganis safer to play.
Mal'Ganis: Combos with almost everything in the deck, buffs most of your minions including Dreadsteed which is the center of this deck, makes your hero immune and has amazing stats. Try to bait out silence/removal with other cards before this comes out (a big Void Terror can help with this.)
Sea Giant: A one of as a late-game finisher. You will almost always have a good sized board so it should be relatively cheap in the mid-late game.
Note: This deck is extremely susceptible to Mass Dispel, but since Priests rarely play it, its a rather poor reason not to play this deck. Although the meta will change and it might become an auto-include for Priest, so if you are up against a Priest just don't over-extend by playing both Dreadsteeds at once and you should be fine.
Edit: Having been playing this deck since TGT came out I have made some changes to the list to try and help against faster decks. -1 Sea Giant, -1 Anima Golem, -1 Void Terror, -2 Flame Imp, +1 Hellfire, +1 Mortal Coil, +1 Twisting Nether, and +2 Zombie Chow. This allows us to have more early game choices that do not attack our own life total (Zombie Chow instead of Flame Imp) which drastically increases our chances vs. fast-paced decks (Face Hunter, Zoo Lock, Shockadin) and makes our transition to mid-late game easier because our life totals are over-all higher. I removed Anima Golem and Sea Giant because they were too often dead cards. By the time either of them were of moderate value you were either setting up for lethal or were about to lose, and in either case Mal'Ganis would help you better anyway. Twisting Nether is a somewhat experimental pick, as a one-of is gives late game board-clear that is ultimately beneficial to us by sacrificing the enemies board, but leaving our Dreadsteeds intact. Hellfire is great for early-mid game board clear against fast decks, and is largely ineffective to our board (Dreadsteeds and Imp Gang Bosses make our board rather resilient). I am currently considering removing Nerubian Egg and Void Terror, due to them being rather iffy at best. When you are ahead on the board they work wonders , but when you are behind neither of them apply the pressure that is needed to help catch up.
If anyone else has any suggested changes please feel free to message me!