Hearthpwn

Do you know the last thing people think of when they hear "Priest"? BURST. I even once heard a caster say, when talking about this subject, "Someone should just throw a Leeroy into a Priest deck just to surprise people on ladder."

Well, I tried putting in Leeroy, but it turns out mind blast is enough on it's own most of the time, and it ignores taunts! I made a pre-ungoro version of this, but now that Lyra has been added, it's gotten even better!

STRATEGY
General strategy is to draw a bunch of cards to make your Drakes huge using the Circle Combo (discussed below). This is the dream scenario, but with Crystaline Oracle and Netherspite Historian, you often don't even need the full Circle combo as each of these cards replaces itself. The most important card in this deck (believe it or not) is Twilight Drake. Yes, Drakonid OP is stronger on its own, but Twilight Drake comes out a turn earlier and really helps control the board while Drakonid OP is used for value. 

COMBOS
The Two-Turn Kill Combo:
Turn 1: Alextraza the enemy and either hit face
if your HP is relatively high just go face with everything you have on board. If you are a little low, make a few trades to protect your face
Turn 2: Mind Blast + Mind Blast + Holy Fire
(You can also just hit them in the face if they don't clear your stuff haha)
tip: This is what got me started on this deck at first, but it turns out, you rarely need to actually pull this off to win

The Circle Combo:
Wild Pyromancer + Northshire Cleric + Power Word: Shield + Circle of Healing
This combo will draw you one card for every minion on the field that isn't killed by the first spell (PWS, The Coin, even Mind Blast in a clutch situation). If you get this full combo off early, you are in a great spot to win the game, but it is still powerful later on as well. It's important not to burn you Clerics on turn 1 or 2 for no reason. Make sure you are always going to get value out of them every time you play them. The only exception to this is if you have two in your hand and want to drop one early or maybe against Pirate Warrior/Hunter since they have a lot of turn 1 1-hp minions that Cleric can just trade into.

The Auchnai Circle Combo:
Literally just Auchenai Soulpriest and Circle of Healing for a 4-mana, four damage AoE. One of the most powerful AoEs in the game!
tip: Auchenai can also enable your hero power to add an extra 2 damage on top of x2 mind blast if you have 10 mana

The Lyra Combo:
Honestly, the Lyra combo is just Lyra plus one of the cheap spells in this deck (or even The Coin!) With just one spell, Lyra can go off on a chain of the spells she generates. There may not seem like a lot of spells in this deck, but you really only need one to get amazing value out of her.

REPLACEMENTS
You can probably replace Alexstraza with another big dragon, but Alex is so versatile as either a clutch heal, or a form of surprise burst.
Holy Fire + x2 Mind Blast = 15!
Most of the time though, I don't even use Alex as a finisher, as T. Drake, D. Operative, and Book Wyrm do such a great job of controlling the board and are all great value cards.

Lyra is not essential to this deck, but it does add a ton of value as many of the early game cards such as Circle of Healing and PWS are very cheap and begin the Lyra super-cycle. With just one of these, Lyra can go off on a chain of spells like a discounted Gadgetzan Auctioneer, and card economy is key to this deck.

Dragonfire Potion and Primordial Drake are interchangeable I think. If you have two of one, but none of the other, that should be fine. Just be aware that replacing one of them leaves you with less options later in the game. Holy Nova is another option as it has synergy with both Pyro and Cleric.

The rest of the cards are of Rare or lower rarity, so you shouldn't need to replace anything else unless you just want to try something else.

FINAL REMARKS
Enjoy! I would love your feedback or questions about general strategy. I have found success with this deck against all decktypes in the meta, and I am still looking for ways to improve it.

P.S. - Freeze/OTK Mage may give you problems (due to Ice Block) if you don't get aggressive early, but the important thing in that matchup is to value face damage on them. Make an strong push with your mid-game dragons and they won't be able to draw their deck and deal with multiple 6-9 HP dragons at the same time.