This is my version of a midrange demonlock. The deck works very well, being able to defend itself against aggro, while also having the power to quickly kill opponents. During the early game, the goal is to build a board of small demons and buff them with Bloodfury Potion or Demonfire. Entering the midgame, your powerful midrange demons come into play. A very strong combo is turn 4 Voidcaller into turn 5 Demonheart. This turns the voidcaller into a threat on its own. Aggro or faster midrange decks are often not able to deal with the buffed Voidcaller AND the summoned demon (ideally Mal`Ganis). Against control decks that manage to survive until the late game, Bloodreaver Gul`dan serves as a finisher. Most of the time, your opponents will have to use their removal and boardclears before this stage of the game to deal with your buffed demons. If your opponent has boardclears left, your resummoned voidcallers serve as a backup.
Individual Card Choices
Demonfire / Bloodfury Potion / Demonheart:
The buff spells give you fast damage, turning even small demons into large threats, and can function together with Mistress of Pain as a win condition against aggro, because a 4/7 or 6/9 lifesteal minion is often too much for them to deal with.
Great card with aggressive Statline, you can even play it without a demon on the board just for the stats, but insane in combination with Imp Gang Boss (Turn 3 Imp Gang Boss, Turn 4 trade into Crystalweaver to get two buffs) or a board of smaller demons.
Demonwrath / Hellfire:
Necessary due to the amount of token decks.
Sylvanas is aa flex spot, you can also play Black Knight to counter the Litch King, or Felfire Potion if you encounter many taunt druids.
Crazed Alchemyst can be included over Dark Peddler if you encounter a lot of Doomsayers.