Hearthstone just rolled out its first ever Tavern Brawl event, which features premade decks squaring off against each other. The event will change every week, and as of today, it's Ragnaros vs Nefarian. Each big bad has their own unique cards and hero powers, drawn from their solo adventure counterparts, giving players a break from the usual format.
You can get a look at a full round from start to finish above, where I play Ragnaros, who starts with 60 life and has minion cards that revolve around lower costs for taking damage. In the end, I wasn't able to take down Nefarian's Razorgore, who slowly gains power every turn he's alive. I definitely should have addressed that earlier.
Either way, this is what you can expect! I have to say it's pretty damn fun, as it's very easy to just jump into and play -- I'll definitely be doing it at least once or twice a week. As a word of warning, you'll need at least one level 20 hero to enter.