Goat Simulator's "MMO" expansion went live on Thursday, and evidently it features the Easter egg above: a Diablo 3-themed area that takes you to a "Cow Level" similar to the one in Diablo 2.
To get there, one uses a catapult to fling himself into "Twistram." It's a mock-up of the village in Diablo 3, complete with the game's music (and gloomy lighting, of course.) Then, wreck shit in the Cathedral to find access to a subterannean passage. There, your goat will find a glyph through which it may pass to the "Not So Secret Cow Farm." The video above shows it in detail.
Of course, once there, the anthropomorphic cow herd wages holy war on the goat, blasting away with lazer beams from their eyes. The cows can be killed (check out the health bar overhead) and you can't, but there are simply too many, so it doesn't appear you can do anything in this level other than run around and have a chuckle.
"The Secret Cow Level" was part of Diablo 2. "Whimsyshire" was an Easter egg in Diablo 3, a much more brightly colored level that featured deadly unicorns, sunflowers and teddy bears. This appears to blend the scenery of the latter with the hostile population of the former.