His hero power summons a 5/5 darkspawn at the start of his turn.
Use your 3 candles wisely. The candles start in your hand and is a 0 mana spell that destroys all darkspawns. it's best if you use a candle to clear all darkspawns before turn 7 or he might psychic scream them into your deck. this is bad for you because the darkspawns are 5 mana vanilla minions which will disrupt your draws.
Turns out, totem of the dead (treasure) + ancestral spirit + Kel'thuzad is preeeeeeeeeeeety good. It's still a really close game though.
If you manage to pull off the combo stated above, using grumble, worldshaker to protect some of your Kel'thuzads is a good idea too.
Tips for The Darkness boss:
His hero power summons a 5/5 darkspawn at the start of his turn.
Use your 3 candles wisely. The candles start in your hand and is a 0 mana spell that destroys all darkspawns. it's best if you use a candle to clear all darkspawns before turn 7 or he might psychic scream them into your deck. this is bad for you because the darkspawns are 5 mana vanilla minions which will disrupt your draws.
He has lots of clears. He used 2 twisting nethers, 1 entomb and 2 psychic screams against me. I think it's safe to assume he has one more entomb in his hand.
He has 2 mind controls also which can punish you severely.
The rouge secrets he used against me are sudden betrayal and evasion. Both are quite easy to play around.
proof: