Hearthpwn

I'm pretty sure this (the 4 hero powers in the DB) is part of the actual mystery AND has to do with rokara adventure, here is why : 

- Obvious: the 4 hero powers have ???, are new (like the achievement), and they are 4 (like the steps).

- they are noted warrior, and Rokara is a... warrior!

- everyone can do it and the adventure is free and require no collection specific thing

- the first one in posting order should be linked to the step of Rokara fighting at the goblin oil site since it looks like an excavation thing (sorry english is not my mothertongue). But let's keep in mind post order does not mean actual order of steps, correct me if I'm wrong. It doesnt look like a key to me, but I might be wrong.

EDIT : I just did the adventure again and I wonder how I remembered a fight at the oil site in the barrens, there seems to be none, my bad.

- The 2nd one posted is a map, as said before seems very "puzzle like".

- I assume the boards on the adventure is/are FIXED (ie not random), and many pointed out that with Celestion post, at least one step might require board-interactions). That would ensure the "doability" for everyone from Blizz perspective.

Those 2 last points are very less clear but I'll post them just in case:

- The 3rd one posted make me think of the step of the adventure near the wailing caverns. A bit of a stretch I admit.

- Another point that is a bit of a stretch : as someone pointed out, the feather on top of the adventure is red, like a phoenix.

Maybe another piece of puzzle:

"Descent of Dragons was merely a set back!" is the quote-text of the card Kael'thas Sunstrider. This card was released with Ashes of Outland (year of the phoenix) and is available to all players (Systems & Initial Game Designer „Celestalon“ twittered "you can all solve it").

Descent of Dragons (year of the dragon) is indeed "a set (expansion) back".

Now we are in the year of the gryphon with Barrens. Maybe you have to play three spells in a row, one of every expansion!?

I noticed the quill in Book of Mercenaries is red like a phoenix feather. Maybe that has something to do with it? 

It certainly seems to have Kael'thas' three orbs on it. But that was a big Ashes of Outland card and nothing to do with the Barrens, so it probably can't be linked with the casting of 1-mana cost third spells.

nothing to do with Barrens, yes, maybe.

but read the card name, "Phoenix"

Kael'thas was a card from "the year of the phoenix". so I believe thats the connection.

Can you claim the new portraits already?

I have the achievement finished, but no portrait claim is possible.

No still bugged

Only made an account for this riddle!

The first thing I think of when I read pheonix is obviously Al'ar, Kael Thas precious pet.

It's flavor text reads "Week #679: Still no mount..."

Maybe those numbers are another hint?

On second look the runes on the cardback itself look to have lopsided 5s and IV, 

So we would have the numbers 4, 5, 6, 7 and 9

I do not think the cardback would have some kind of a secret code in its picture, since it was made for a cardback competition by a participant, and not by blizzard itself.

The secret thing is pretty interesting.

maybe there are some hints on the cards descriptions ?

Getting the golden Transfer Student cards had to do with game modes and some other older secrets were board centric, so might be a combination of the 2. Never was able to solve the Witchwood board secret either.

I don't think it's on the board, because that would be too much dependend on chance.

I'd say we'll have to look for clues/hidden items on the reward-track map.

Just a guess...

On the Barren's board, there's a hut in the top right-hand corner with 5 drums on the roof, my guess is that there are 4 different drum beats in the trailer or Barrens which you have to recreate on the drums on the board to get the card back

It's the Heart of Te Fiti.  Moana confirmed as a new character skin for Valeera.

What if it has to do with the book of mercenaries? When you select a hero power you get unique dialogue with that hero. Almost like a progressing mini-story within the story. And its 4 heroes. So maybe complete the book of mercenary while only ever choosing one specific hero power from start to finish?

That's an interesting idea. Definitely seems plausible.

I guess one way to tell if you're on to something would be to do a run with all the same hero power and see if there is progress on the achievement.