It looks like Blizzard has hidden a secret level in Diablo 3. By gathering various items across your journey you can unlock Whimsyshire, a hidden level filled with happy clouds, rainbows, unicorns and everything else you wouldn't ever expect to find in Diablo 3.
Of course, that was probably Blizzard's goal with Whimsyshire which is quite possibly a response to early criticisms of the game's aesthetic in which fans called Diablo 3 "too colorful".
So how do you travel to this magical land of Whimsyshire? You'll need to first acquire all of the following items:
- Black Mushroom which spawns in cathedral level 1 from Act 1
- Leoric's Shinbone which is located randomly in the fireplace in Leoric's Manor Act 1.
- Liquid Rainbow is found by traveling to Oasis, head SW until you find a small path leading to a watery clearing, save the rare spawn Zaven The Alchemist from the vultures, explore the mysterious cave he provides access to, and locate the rare spawn Mysterious Chest. If either the Mysterious Chest or Zaven don't spawn, then the game will need to be remade.
- Wirt's Bell is sold from Squirt the Peddler in Caldeum Bazaar in Act 2 for 100k gold.
- Gibbering Gemstone drops from Chiltara a random mob in the Caverns of Frost Act 3.
- Plans: Staff of Herding drops randomly from Izual on Act 4.
After acquiring all of the necessary items you can build the Staff of Herding at the Blacksmith. Next locate the Ghost of the Cow King (a reference to Diablo 2's secret Cow Level) and he will open up the gate to a "level of sparkling happiness and rainbows".
According to the Diablo wiki, the secret level mobs are balanced to be equivalent to late Act Four meaning low level characters, even on normal difficulty, will likely be slaughtered upon entering. In addition to the items you will find there, you will also receive a feat of strength, a banner, and a sigil.
Don't expect Whimsyshire to be a major focal point in Diablo 3; the developers didn't want to recreate another Cow Level which rewarded players a little too handsomely for their adventures there. Still, it's still a nice little touch from Blizzard to pay homage to the fans and to Diablo 2's Cow Level.