Combine the two items, and a portal into the cow level will appear. Wirt’s leg is fittingly found on Wirt’s Corpse, which rests in the northwest corner of Tristram. Then, combine a Tome of Town Portal with Wirt’s Leg in the Horadric Cube while in the Rogue Encampment.
The level exists on Normal, Nightmare, and Hell, but you need to kill Baal on each difficulty to access their respective cow levels. To create the portal into the secret cow level, you must first beat Baal on your current difficulty. How to access the secret cow level in Diablo 2: Resurrected
