Hearthstone's mysterious fourth button leads to a new challenge mode.

Hearthstone patch 2.7 adds a new gameplay mode called Tavern Brawl. This is the "fourth button" Hearthstone mode Blizzard was teasing earlier in the week.

A competitive mode, Tavern Brawl imposes various restriction on duellists - the use of a preset deck, say, or restrictive deck building guidelines. It sounds tough, and players must reach level 20 before they can access the new mode.

Tavern Brawl will only be available from Thursday to Monday each week. Entry is free, and you'll score a free card for your first win every week during the new mode's launch period. Rewards are expected to rotate regularly.

In addition to Tavern Brawl, the update adds in $10 cosmetic Hero skins and the chance to assign custom card backs to specific decks. The new Heroes will be sold for £6.99 if you purchase them through the in-game shop of the desktop client, or £8.99 if you buy them via an iOS device.

The patch has not been dated by Blizzard said on Battle.net that the first Tavern Brawl will kick off "mid June", so it can't be far off.