Hearthstone's "Felfire Festival" kicks off tomorrow with the release of the 17.4 update. The update adds 17 Pirates (a new minion type) to the Battlegrounds mode. The update also adds three dynamic new heroes, retires a fourth, and sets the stage for next week's "Trial by Felfire," a free solo adventure on Hearthstone.

First up is the solo adventure: this five-chapter story, which begins June 17, will see Aranna Starseeker join forces with Illidan Stormrage and his Demon Hunters to face off against Rusted Legion mastermind Mecha- Jaraxxus, the mastermind of the Rusted Legion. Unusually for a Hearthstone adventure, all chapters are released at once.

"It's pretty epic," Blizzard states, which is not the most specific sales pitch I've ever seen.

A week later, on June 24, a new challenge mode will launch that will give successful players a Golden Caelus Sunstrider card. The Burndown Tavern brawl will also return: players will enter the brawl with a random deck, and after losing, they will be handed their opponent's deck and try their luck again and again until one deck prevails. Then, on July 1, the burndown ends and the Rumble Dome begins: a large crowd enters, but only one leaves.

A new Legendary Quest line, with nine card packs up for grabs, will also begin on June 17.

Battlegrounds will add a minion pool move to the Felfire Festival: At the start of each game, five of the current six minion types (Beasts, Demons, Dragons, Mechs, Marlocks and Pirates) will be available in the recruitment pool. Also, in the interest of fairness, heroes designed to synergize around a particular minion type will not be playable if the corresponding minion type is not available.

Here are the new pirate heroes mentioned above:

For players who have unlocked Battlegrounds Perks, Aranna will also be added as an early access hero on June 16.

And guess who's back?

The Rat King was changed to start with a random minion type instead of the Beast King every time, Professor Putricide was removed from the hero pool, and Holy Mackerel was removed from the minion pool because he is as powerful as a small fish.

Speaking of minions, only one new pirate appears, a familiar one from the main game mode.

This update also fixes a couple of bugs, one of which is that your opponent's mana crystals will show up in the sidebar during your opponent's turn in Mobile, and the other is that Zefries the Great will not provide Fengjutsu to non-target minions if they have high Fengjutsu value The other is that Zephryth the Great does not provide Feng Jutsu to non-target minions if they have a high value. More details can be found at playhearthstone.com.