Hearthpwn

 Dragon Lord Velen has gotten me to legend 3 times (capping at rank 51 legend in S12), and I have since put almost 4,000 hearthstone games in with this archetype. It's not T1, it's not the next "easy legend" list, but it's strong enough to keep me at a solid 70% lifetime win-rate with the list, and I consistently use it to climb to rank 5 every season on the wild ladder.

Dragon Lord Velen is a tempo deck at its core. The deck establishes an early board presence via Northshire Cleric, Twilight Whelp, Netherspite Historian, and Wyrmrest Agent, maintains this tempo into the mid-game via Velen's Chosen and Twilight Drake, and finishes off the game with surprisingly long reach granted by Holy Smite, Mind Blast, and Prophet Velen.

You may notice that this deck runs no single target removal, and only runs AoE in the form of Holy Nova. As per its tempo nature, this deck establishes and maintains board control via minion combat. Dragon Priest is especially adept at this as its minions are naturally high stated for their cost, and the priest hero power lends itself towards keeping minions around. If you lose the board, you typically lose the game (unless you have set up the combo kill, which we'll get to).

This deck is built around a magic number at which it can consistently kill its opponent: 15, although it has a number of damage ceilings, which are as follows:

Combos requiring Emperor Thaurissan to reduce costs:

36: Prophet Velen + 2xMind Blast + 2xHoly Smite + Bloodmage Thalnos (5 reduced)

30: Prophet Velen + 2xMind Blast + 1xHoly Smite + Bloodmage Thalnos (4 reduced)

28: Prophet Velen + 2xMind Blast + 2xHoly Smite (3 reduced)

24: Prophet Velen + 2xMind Blast + 1xHoly Smite (2 reduced) OR Bloodmage Thalnos (3 reduced)

20: Prophet Velen + 2xMind Blast (1 reduced)

Combos that require no cost reduction:

18: Bloodmage Thalnos + 2xMind Blast + 2xHoly Smite

16: 3xShadowreaper Anduin's hero power + 2xMind Blast

15 damage combos:

Blackwing Corruptor + 2xMind Blast + 1xHoly Smite

Bloodmage Thalnos + 2xMind Blast + 1xHoly Smite

Any 1 or 2 drop minion + 1xVelen's Chosen + 2xMind Blast + 1xHoly Smite

Any damage combo that does less than 15 should be obvious, as it will involve 3 or less cards

These are tips, and do not apply to ALL situations you will encounter when playing this deck, but they are rules you should try to live by in 99% of matches.

Mulligans:

Mulligan for dragons. If you have a dragon, mulligan for low costed cards with dragon synergy, like Twilight Whelp, Wyrmrest Agent, and Netherspite Historian. If you do not have a dragon in your start hand, mulligan EVERYTHING  -and I do mean everything- to find a dragon. This deck redefines the term garbage when it gets a hand stuffed full of "better with a dragon in hand" cards, and no dragon in hand. With a dragon, its an unstoppable juggernaut in the right hands.

Rules:

1) NEVER, and I do mean NEVER, play the last dragon in your hand unless you are out of dragon synergy cards in your deck.

2) while the first Holy Smite can be used as removal in turns 1-3 against aggressive decks, NEVER burn the second Holy Smite as removal unless you absolutely have to in order to prevent yourself from dying. Against classes that are prone to healing, such as priest or jade druid, do not use Holy Smite for anything but combo damage.

3) If you are still fighting for board control, prioritize playing high stat minions over high value minions (EG: Wyrmrest Agent over Netherspite Historian). If you have solid control over the board, instead prioritize playing the value minions.

4) Don't play Shadowreaper Anduin against aggro decks. It doesn't work out for you.

5) Holy Nova is worth more than Blackwing Corruptor due to its synergy with Northshire Cleric. If you only need to remove 1 or 2 minions of your opponent's, try using the Blackwing Corruptor unless you can abuse both the damage and healing that Holy Nova provides.

6) Save Bloodmage Thalnos as a cheap way to boost Holy Nova to dealing three damage (comes in handy surprisingly often vs aggro), or as a way to boost your combo ceiling (surprisingly impactful vs control), depending on what type of deck you are facing up against

7) On the play, try to save The Coin to boost the health of your Twilight Drakes. It matters a lot more often than one might think, and establishing a solid early board presence can be the difference between winning and losing games.

It goes without saying that this list does not function without Prophet Velen and Emperor Thaurissan. This means I am assuming that you have the Blackrock Mountain expansion's first wing if you desire to play this deck. I am also assuming that you have access to all common/rare rarity cards, and all basic set cards.

Bloodmage Thalnos

Replace Bloodmage Thalnos with Tainted Zealot. They are of similar cost, both serve the cheap spell damage function required to boost this deck's damage ceiling, and while Bloodmage Thalnos cycles himself, Tainted Zealot sticks around pretty well, making it a good target for Velen's Chosen.

Shadowreaper Anduin

Shadowreaper Anduin is a niche card I'm still testing out. Aside from Emperor Thaurissan and Blackwing Corruptor, this deck runs no 5 attack minions, which means that Shadowreaper Anduin's battlecry rarely back-fires (watch it, it still can). It's good as mass removal for large minions, which is very good considering this deck lacks removal for big creatures, but is also good to whittle down opponents just a little bit to get them into combo range. Because it's such a unique card, you can try Dragonfire Potion/Lightbomb for the removal effect, or could swap in a Drakonid Operative to go for OP tempo swings. I used to run Alexstrasza in this slot, as she can provide useful healing against aggro decks, and can knock control decks down to that magic number 15. She increases the dragon count, but the drawback is you can only ever play her when you have a "free turn" to set up your combo, which is a big ask of a tempo deck, especially since you already needed to find a "free turn" to play Emperor Thaurissan to help set up your combo. This is a tech slot as far as I am concerned, so feel free to fiddle with it, although the go-to replacement for Shadowreaper Anduin is Alexstrasza. Just note that Shadowreaper Anduin is the whole package, and each of these replacement cards will leave you with something missing.

Blackwing Corruptor/Twilight Whelp/Netherspite Historian

Let's say you don't have the adventures these cards came in. They are common/rare rarity, and therefore quite cheap to craft. As such, I recommend either crafting them with dust, or acquiring the adventures they come from, as adventures are some of the best investments that hearthstone has  to offer. Blackwing Corruptor CAN be swapped for Drakonid Operative if you need to, but the other two are such powerful cards that they cannot be replaced without severely impacting the deck's winrate.

Wyrmrest Agent/Velen's Chosen

These two are from sets that have rotated out of standard, but they, like their adventure based counter-parts, are integral to the deck's strategy. As such, craft them with dust. Velen's Chosen can be replaced by Blackwing Technician, but it heavily damages the deck's consistency and combo ceiling, as that spell damage is extremely relevant for this deck. Wyrmrest Agent is irreplaceable.

Every other card in this deck is from the basic/core set, and is at rare rarity or under. While this list isn't extremely expensive as far as dust goes, it is also not designed to be a budget deck.