There is one more thing worth mentioning too that is extremely important. I'm not entirely sure what causes this bug but I have had it happen enough times on enough characters that it is worth mentioning. Or complete a quest in the next difficulty then come back to the difficulty you beat the game on and try again. It isn't uncommon for the combine to fail sometimes, usually you have to log fully out of the game then back in if this happens to you.

Wirt's Leg + Tome of Town Portal = Secret Cow Level Once you've done this you'll need to combine Wirt's Leg in a Tome of Town Portal in the Rogue Encampment to open the portal to The Secret Cow Level. When you are logging back in make sure you choose the same difficulty that you beat the game on. In order to reach this level you will need to complete the game (defeat Baal) and log out or take the Destruction's End portal so you see the screen with Deckard Cain.

Secret Cow Level is one of the most famous areas in all of Diablo 2 and is a bit of a meme in Blizzard games. Hell Immunities: None (Only Boss/Champ packs) How to Reach: Rogue Encampment > The Secret Cow Level Secret Cow Level Farming Location - Diablo 2

Diablo 2 normal cow level