According to players on the WoW subreddit, reaching the cow level involves killing treasure goblins throughout the Broken Isles. Here’s what it looks like, courtesy of Wowhead:
Cows! If you can’t watch the video, rest assured that they’re everywhere, mooing and dying, as cows do.
Killing the Cow King will get you a twelve-string guitar item that plays the Tristam theme from Diablo. If your nerdy youth was spent in the warm, sporadically moo-ing embrace of Blizzard games like mine, you probably just got embarrassingly excited about a glorified button that plays an MP3 file.