The Darkness is definitely the most controversial card from the new expansion, but that also makes it the most interesting one.
Here is what happens when you play this card. First, it becomes dormant, which means that it can't be silenced, targeted, or destroyed. Then, you need to wait for the three candles that go into an opponent's deck to get activated at the draw. And finally, as soon as all three candles are drawn, The Darkness will awaken by turning into a 20/20 minion.
It may take a while before that happens during the game, but when it does, you will be able to attack for 20 damage, and that's basically game-winning.