The goal this deck looks achieve an OTK finish for 27 damage with Malygos, Mind Blast 2x, Holy Smite. To achieve this the mana of all those cards has to be reduced by Emperor Thaurissan. This isn't neccesarry as you can always play Malygos turn 9, and if your opponent doesn't have any removal then the game will usually end the next turn. Emperor Thaurissan can usually bait out removal himself.
Why not Velen?
This deck shares a resemblance to the Mind Blast Priest that Combo Prophet Velen, Mind Blast 2x, and Flash Heal with Auchenai Soulpriest, but I think the deck is more reliable. The cause of this reliability is found in the dragon synergy that Priest has. With great early game minions such as Twilight Yelp and Wyrmwrest Agent gives Malygos some purpose if it gets stuck in your opening hand. Malygos also creates more value out of smite that you don't get with Velen (7 Damage instead of 4), and smite is more flexible than Auchenai-Flash Heal, as it can handle early game minions if need be.
At times when Emperor Thaurissan seems to be at the bottom of you deck, it may be necessary to play Malygos a turn early. If he survives you can also use Holy Nova instead of Holy Smite to finish the combo.
Aggro- If you can survive the initial onslaught of aggro decks then the game will look pretty good for you because they run out of steam . Then you can just outpace them with you hero power.
Mid-Range- As long as you can maintain board control or at least survive till the late game, you should be able the finish your opponent off with a combo.
Control- Most likely your best chance to win the game as these match ups usually go for many turns allowing time to set up the combo. It's important to save your entomb for their largest minions for major value and more late game.
C'Thun- Against C'thun decks this deck seems to fair pretty well. As long you can handle the Twin Emperor Vek'lor, which can't be removed with shadow pain or death, then your odds look fair. C'thun himself doesn't pose much of a threat unless your at low HP which usually doesn't happen against C'thun decks by turn 10. C'thun can be easily handles with Shadow Word: Death or Entomb, or if you have the combo and can just end the game.