There is actually a secret unicorn level. You will need a set of items to active it. There are also some steps that need to be taken.
Here is the list of items.
Black Mushroom – Found rarely in a room on Cathedral level one.
Wirt’s Cowbell – Act II vendor sells it for 100,000 gold.
Liquid Rainbow – Spawns in the Mysterious Cave in the Mysterious Chest in the Oasis, Act 2. Save the old man and he will open the chest.
Gibbering Gemstone – Random drop from Chiltara in the Caverns of Frost, Act 3. It’s in the Ballistae fight area. If you get Ice Caverns exit out try again. The mob is random.
Leoric’s Shinbone – Found rarely in the fireplace in Leoric’s Manor.
Plan: Staff of Herding – To teach the recipe to Haedrig. Drops randomly from Izual, Act 4.
Can someone help fill in the blanks? After you get all the stuff, where do you go? I heard you had to go to some dead cow somewhere.
Here is a video for the secret unicorn level.