First he plays a Howlfiend which he then gives to the opponent using the spell, Treachery. Howlfiend has good stats for its mana cost, but it has the downside of discarding a card from the owner's hand whenever it takes damage. Next he plays an Imp Gang Boss which has the effect of spawning 1/1 imps whenever it takes damage. Then he plays a spell called Defile. This spell only does 1 damage to every minion, but whenever a minion dies, it's recast. So the 1/1 imps from the Imp Gang Boss keep it going until everything dies. This entire time, the Howlfiend has been taking 1 damage at a time from the Defile. The discard effect causes the Priest to discard his entire hand of 6 cards. Then he plays a summoning portal which reduces the cost of minions by 2 mana. After that he plays a legendary minion called Glinda Crowskin which gives all minions that you play Echo. If a minion that has Echo, you can play multiple of them in a single turn as long as you keep paying the mana cost. Using the Echo effect from Glinda and the cost reduction from the Summoning Portal, he plays 5 Lore Walker Cho. This minion has the effect of giving any spell you or the opponent casts to the other player. So if the Priest cast any spells, the Warlock gets a free copy of it into his hand. Finally the Warlock plays a spell called Power Overwhelming twice. The effects don't really matter, but what's important is that the spell can ONLY be cast on friendly minions. Since the Warlock has 5 Lorewalker Chos, the spell is copied to the Priest's hand 5 times for each spell cast. 2x5 is 10, and since the Priest has no cards in his hand, his entire hand becomes copies of Power Overwhelming. With 10 cards in the Priest's hand, his next draw is discarded, because you can only have 10 cards in your hand. But since the Priest can't play a minion, he has no way of casting the Power Overwhelmings that are filling his hand. Therefore he's unable to play anything and just dies slowly to the Death Knight Warlock's hero power and Glinda.