I am sure you are really curious to find out how to unlock Whimsyshire or how to enter Whimsyshire in Diablo 3 and I will share all the details below, but be warned that there are some spoilers included which require us to take you through all 4 acts, so it’s up for you to decide if you read or not.
If your answer was “let me see”, here is how to enter the Whimsyshire secret level in Diablo 3:
Basically you need an item, the Staff of Herding which should be crafted by a blacksmith using 5 secret ingredient and a plan. Then, the item must be taken to the Cow King’s Ghost, on Old Tristram Road, near that pit with the mysterious red glow from within. After that, it will change into a rainbow and you’ll be able to click it and enter the dungeon.
Here are the required ingredients for the Staff of Herding, required to enter the Whimsyshire secret level:
Black Mushroom – Found rarely in a room on Cathedral level one, in early Act One.
Leoric’s Shinbone – Found rarely in the fireplace in Leoric’s Manor, in late Act One.
Wirt’s Bell — An Act II vendor (Squirt) sells it for 100,000 gold on her Potions and Dyes tab.
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.
Plan: Staff of Herding – To teach the recipe to Haedrig. Drops randomly from Izual, Act 4.
Good luck getting all the items and unlocking the Whimsyshire secret level in Diablo 3!
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Source: Diablo 3 Wiki