To unlock Whimsyshire you’ll have to pick up a number of secret items:
Black Mushroom – Tristram Cathedral, Act 1
Leoric’s Shinbone – Leoric’s Manor, Act 1. You can find it in the fireplace in the room to the right.
Wirt’s Bell – From Squirt, a merchant in Act 2. She’s a little girl in the Caldeum Bazaar.
Liquid Rainbow – Found in the Mysterious Cave in Act 2. You’ll have to open a “Mysterious Chest” to get it. The cave is located in the Dahlgur Oasis.
Gibbering Gemstone – A rare monster called Chiltara in the Caverns of Frost, Act 3.
Blacksmith Plans, Staff of Herding – Finally, you’ll need to get the blacksmith plans for Staff of Herding. Izual has a chance of dropping them.
Once assembled, just head on over to Old Tristram and look for the skeleton of a cow. A ghost will then speak to you and unlock Whimsyshire.
Gameplay of Whimsyshire