Kill Coren Direbrew.
Coren Direbrew is, for the most part, a single-phase fight with a soft enrage (adds appear at ever-increasing speed as Coren's HP is depleted). The encounter tests players on their situational awareness and DPS prioritization. Since Cataclysm, the only means of accessing the boss is either through the Looking For Dungeon tool or speaking to Battered Brewer in any faction and sanctuary capital city. A player can kill Coren as many times as he or she desires for a chance to win loot from his corpse but will only be awarded Keg-Shaped Treasure Chest for the first kill of the day.
Notable loot from the chest:
Notable loot from Coren Direbrew (2012 & 2013):
Coren Direbrew exists in a special version of Blackrock Depths which may only be accessed via Looking For Dungeon. Players will need to be at least level 89 to queue for the holiday boss encounter. It should* be still possible for characters lower than level 89 to get around this restriction by having five friends or willing PuG players create a party, queue for Coren Direbrew, and enter the dungeon. One of the party members must be a Warlock! One of the DPS party members must drop from the instance group. The party leader then invites the lower-level character to group and that character accepts and joins the group. The warlock, with the aid of others, casts a Ritual of Summoning. The waiting character is then summoned into the instance. Profit.
*I guarantee nothing.
Brewfest last year was extremely close to the launch of the Mists of Pandaria expansion. As such, I didn't manage to make level 89+ in time to experience the updated encounter. (No, it didn't occur to me to try to get summoned in. Still kicking myself for it.) Once I have, I will fill this section out more appropriately. Please see noteworthynerd's comment here in the meanwhile.
I experienced the fight, and noteworthynerd's comment is still accurate. Ignore the small adds and drink the brew that may be thrown into your inventory by either Ursula Direbrew or Ilsa Direbrew. Don't stand where the mole machines come tunneling up from the ground; they interrupt casting and knock you back. Do not stand within range of Coren Direbrew's Direbrew's Disarm, including the tank! (It disarms you for a few seconds and knocks you down.) DPS should just focus on killing Coren.