Nightmarish Staff of Herding
From Diablo Wiki
The staff has only one use, to open a portal to the secret level Whimsyshire. Having this item in your inventory allows for you to access Nightmare Difficulty level.
This can be crafted after obtaining the plans for it from Gorell the Quartermaster in Bastion's Keep Stronghold. You must be in Act IV Nightmare + have Prime Evil quest to buy this!
Merchant sells the plans for 20,000 gold. The plans require Staff of Herding and 200,000 gold to craft.
This staff upgrades to the Hellish Staff of Herding.
 Purchase From
Gorell the Quartermaster, Bastion's Keep.