The cow level was a secret location in Diablo II where players could slaughter cows and gain loot from high level chests. Sadly, that adventure did not reappear in Diablo III until now. Blizzard has updated Diablo III on PC, PlayStation 4, and Xbox One with a cow level accessible by killing the elite “Herald of the Queen” enemy.
Once killed, a portal will appear to bring you to the cow level, complete with hordes of Infernal Bovines, a new event, and a Cow Queen boss to defeat. You can find some gameplay of the level below. Please note that this will only last from May 15-21, 2015.