It's based on the Argent Tournament event from World of Warcraft, though unlike the Argent Tournament the participants aren't so much here to prove themselves worthy to fight the Lich King and more just to beat each other up for glory and for fun.
These cards cannot be opened in packs but can still be crafted and used in Wild format.
Classic cards can be opened in Classic packs, and have more complex mechanics. Most Basic cards were given to all players for free, with a small handful unlocked by leveling the respective class to 20.
Two omni-present sets of cards that would never rotate into Wild, except a few which are sent to the Hall of Fame each year.
It contains 20 cards, all of which were given for free upon completing the Demon Hunter Prologue.