Diablo 3 - How to get to Whimsyshire (Cow Level)

You need the Staff of Herding.The items required to create the Staff are: Black Mushroom, Leoric's Shinbone, Liquid Rainbow, Wirt's Bell, Gibbering Gemstone. Once you acquire all of the materials you must craft the staff using Plans: Staff of Herding. The plan drops randomly off Izual in act 4, in any mode.