I just played a match with this week's Tavern Brawl. I won and got the card back, but it seemed like there was supposed to be more to it. I ended up killing 11 minions and activating 0 secrets. Kind of a weird thing.

At least the card back looks cool?

There is a pool of secrets and you get 3 different ones each match. If you trigger all 3 secrets, a portal will spawn. Whoever controls the portal at the start of the following turn becomes the Cow King and their deck is transformed into cows. Since the portal is technically a minion you can steal it with Sylvanas, so if you want to become the Cow King it's good to hold onto something like a Sylvanas and Power Overwhelming or Shadow Word: Death combo.

@mike: Whoa! That's pretty crazy. I'll give it another go.