There was rumours for years that a secret cow level, a homage to Diablo, was hiding in World of Warcraft somewhere. There wasn't. But as part of the 20th anniversary celebrations for Diablo, there now is.
To get to the secret cow level (thanks, Eurogamer) you have to find a treasure goblin in one of three places: the Dalaran Sewers, Legion dungeons and outdoors in the Broken Isles. If you kill the goblin before it can finish casting an escape portal, a portal will open to the fabled land of Duskwood.
Inside, you'll be able to charge The Cow King and grab special holiday loot. It's only available to each character once per day, as is standard for WOW's holiday shenanigans. Wowhead have produced a video showing off the whole process, which you can see below.
Not bad, Blizzard. Now I just want them to take it a step further: recreate Tristram and the initial levels of Diablo in WOW. Go on, you know you want to.