Mystery Keg
Source: Quest. For those curious how this item works: Picture the 15 other "Keg" items in the game: you place it and then you can right-click it to get drunk.
The 
Mystery Keg is a Draenor toy obtained through a quest line tied to the Hearthfire Tavern (Brewery) Outpost in Spires of Arak.
Guide awaiting review
Choose the Brewery Outpost
The Mystery Keg requires you to have built the Hearthfire Tavern (Brewery) as your Spires of Arak Outpost rather than the Smuggler's Den. If you already chose the Smuggler's Den, speak to Taskmaster Gornek (Horde) or Lieutenant Willem (Alliance) and pay 5,000g to switch outposts.
Complete the Brewery Quest Line
After building the Brewery Outpost, complete the quest chain beginning with Befriending the Locals, followed by Not Here Not Now, Attempted Murder, Assassin's Mark, The Power of Poison, Extrinsic Motivation, Gardul Venomshiv, We Have Him Now, No Time to Waste, and Standing United.
Place and Use the Keg
Once obtained, place the Mystery Keg on the ground and right-click it to drink from it, which will cause intoxication along with random additional effects that can include being set on fire, frozen, or electrocuted.
Tips
You can recoup 1,000g after paying the 5,000g outpost-switching fee later in the process.
