We already know this week Tavern Brawl but it does not mean that we're not happy to see it coming back!
You just need to pick a class to start a duel, then you will have the chance to "discover" a new class just after the mulligan phase. You will then possess a completely random deck containing cards from both classes you picked. Keep in mind that this Tavern Brawl obey to Wild Standard rules — meaning you will find yourself playing cards that have rotated out.
The class you have picked first is the most important as it determines your starting hand as well as your hero power. However, keep in mind that this Tavern Brawl is precisely made to allow weird-overpowered combos and interactions. Be creative: mix Mage and Shaman to enjoy a nice burst, Warlock and Paladin to swarm minions, etc.
The fact that you can fuse two classes is really interesting, but the huge randomness of this Tavern Brawl makes it a bit frustrating. As usual, you will get a Basic Booster with your first victory.