Cubelock hit the meta game in the Kobolds and Catacombs expansion. The deck seeks to cheat out the two big and impactful Demons it has in the list: Doomguard and Voidlord. This is usually done with Possessed Lackey, which you can then Dark Pact or trade to bring out one of your minions. You can also use the Warlock Legendary weapon, Skull of the Man’ari to pull it from your hand. From here you can use Carnivorous Cube to duplicate it, or the other various copy minions that could be in your deck: Prince Taldaram and Faceless Manipulator.
Cubelock was nerfed by adding more mana to Possessed Lackey and reducing the healing of Dark Pact. It hit some hard times, but still floated around as a viable option for the ladder.
The Boomsday Project brings Ectomancy to the party. This card will summon copies of Demons you control. This could potentially allow you to spawn a couple more Doomguard to finish your opponent, or get some extra Voidlords to wall off your opponent.
Check out an in-depth look at this deck type with mulligans and play strategy via our Cubelock Deck Guide!