Do you remember back when Diablo III was first announced. A rainbow in one of the screens had a legion of Diablo up in arms. Colours? That's not Diablo!!!
Blizzard could not stay away from trolling this fans and that is probably the reason for Whimsyshire - a magical place with pink unicorns, rainbows, sunny skies, and lots of colour.
You can check out a video of the delightful level on YouTube, and if you make your way over to the official forums you can get instructions on how to make your way into this secret level. It requires a few rare drops, spawns and lots of gold, but surely, it's well worth it. Don't you think?