A new Tavern Brawl is now live on Hearthstone servers around the globe and it's a completely new one for the game!
Titled Shadow Reflection, you'll have to build a deck of any size, with the remainder of the 30 card limit being replaced with Shadow Reflection cards. This is the spell card generated by the Rogue Legendary Valeera the Hollow, which transforms continuously into a copy of the last card you played.
A user over at Hearthpwn recommends creating a deck of around 15 cards, and picking cards with a particular emphasis on Mana reduction so you can make more plays than your opponent. They also sensibly warn you to watch the order you play cards in, so you gain maximum benefit from Shadow Reflection.
They also have a handful of Brawl decks which you might like to mess around with. Let us know what's working for you in the comments so we can all bag our first win and get our hands on that Classic card pack!