Hearthpwn

Undead Priest has taken over Frost Death Knight's rule of the Hearthstone ladder. According to HSReplay, it only has unfavoured match up against Control Priest, APM Priest, and Aggro Mage.

IF YOU ENJOY THESE DECKS, POSTS, AND VIDEOS, PLEASE CONSIDER GIVING THIS DECK A THUMBS UP TO HELP ME OUT!!! THANK YOU!

(Want to see the deck in action? Check out the video below for some insane highroll turns/highlights, as well as a Deck Breakdown to help you learn how to break the meta!)

Gameplan:

Aggro Shadow Priest is one of, if not the best, decks in Hearthstone right now. The top legend metagame is warping around countering this deck or playing it optimally. While the best deck is still up for discussion, I believe the best way to play this deck is to focus on a Burn Damage build, instead of Undead synergies with cards like Banshee and Foul Egg.

Why Burn? Because this deck doesn't care about swarming the board with small minions. This deck deals large amounts of damage in waves with cards like Incorporeal Corporal, Shadowed Spirits, Haunting Nightmare, and Brittleskin Zombie. Little minions are great... when you play them at the beginning of the game. Otherwise, they're so much weaker than these larger minions that often do something productive.

Incorporeal Corporal finally sees play in a deck, not only as a "Mindblast on a stick," but due to its synergy with Whispers of the Deep. This combo almost solos any other aggro match up. 

Undying Allies + Shadowed Spirit/Brittleskin Zombie simply wins games if done by Turn 2/3.

High Cultist Basaleph is INSANE if your opponent clears your Reborn Undead minions. I would highly recommend this craft.

Astalor Bloodsworn makes the cut because of its synergy with Voidtouched Attendant to make control decks more favored. It's not always best to tempo your Voidtouched, not just to save for Astalor, but for combos with cards like Shadow Word: Undeath and Shadowcloth Needle plays, which if timed correctly, can be game winning against most of the meta.

Blood DK isn't that bad of a match-up because of this, but Control Priests still do very well as the counter to aggro Priest.

Mulligan:
ALWAYS KEEP: Shadowed Spirit, Haunting Nightmare, Incorporeal Corporal, Voidtouched Attendant, Shadowcloth Needle, Undying Allies

SOMETIMES KEEP: Mind Sear (against aggro, sometimes with needle), Whispers of the Deep (with Incorporeal), Cathedral of Atonement (on coin or with curve plays)

EDIT (3/8):

-1 Void Shard
+1 Najak Hexxen
(You don't need to cut Void Shard if you don't have Hexxen, but this legendary is great to steal taunt minions to set up lethal, or just to steal a big minion in general. With Whispers of the Deep, it makes sense to run Hexxen as one of your few comeback cards.)