The Diablo 3 patch 2.4.3 is expected to drop any day now, celebrating the 20th anniversary of the franchise with the Darkening of Tristram tribute levels. The reminiscing has also spread to World of Warcraft, where players can now no longer say "there is no cow level."
The WoW Subreddit noticed that the event was live and Wowhead relased a video showing it. Apparently, treasure goblins can appear at random in the various game zones or in dungeons, and after killing them, players can take their portal to the new level called Moo-Moo Farm. There players, will kill Diablolical Bovines and Taurens, and eventually kill the Cow King, who will drop a toy guitar. For those unaware, Diablo 2 had a secret cow level, also inhabited by the Cow King.
There have also been reports that Imps in the game now look like mini-Diablos as well.
Blizzard will be rolling out Diablo tributes in their other games in coming days. One is planned already planned for Heartstone. It is likely the level will not last long, perhaps ending as early as next Tuesday when the servers are updated again. So go find that Goblin, and enjoy some incessant moo-ing.