The new tavern brawl is a Diablo feature with cow king

(you can easily get your pack by using a wipe spell on the portal card, e.g. Doom! or Deathwing)

It’s a homage to the Dark Wanderer (Diablo) in Diablo II as well as the Cow Levels, a mainstay of the Diablo franchise

cow-king-unvealed hearthstone-diablo-tristram-special