There was a hidden cow level in Diablo 2, accessible through means which I've long since forgotten, filled with deadly herds of Hell Bovines—literally bipedal cows with spears—ruled by the Cow King. It was purely a novelty, although also a good place to hit up for experience, and so it didn't make it into Diablo 3. Until now.
In a Battle.net post entitled "Debunking the Cowspiracy," Blizzard said the rumors of an invasion of "bipedal, bardiche-bearing bovines" is obviously untrue and flat-out ridiculous. "There are no homicidal heifers. There is no invasion. And, more specifically, there is no cow level," it wrote "Anyone who claims otherwise is full of bull and simply milking this terrible prank for all that it's worth."
However, for the sake of caution, it also advised players to avoid the wilds of Sanctuary from today until the evening of May 21, and to most definitely not enter any strange portals "that may or may not lead to alternate planes of existence." Doing so, it warned, "cud be potentially hazardous to to your health."
The post doesn't actually reveal what's going on, but according to Eurogamer, players wandering the woods may encounter an Elite cow enemy called Herald of the Queen. Killing it will open a portal to "Not the Cow Level," which contains the Cow Queen, as well as a large number of Resplendent and Radiant chests, and a special Event. It's all to celebrate the third anniversary of the launch of Diablo 3, which happened on this day in 2012, and will come to an end at midnight Pacific time on May 21.