The cow level was the most popular place to level up in Patch 1.09, due to the huge numbers of cows and the fact that their slow speed, low resistances, and clumping behavior made them very easy to kill in bunches. They will still be able to enter the portal if another player creates it. If someone kills the Cow King for the first time on the current difficulty, all players within the Cow Level will forever be unable to create another portal to the Secret Cow Level on the same difficulty again. A rather special cow, named The Cow King, always appears in the level. The Secret Cow Level is a small area, full of Hell Bovines, bipedal cattle with polearms. If playing classic, that means killing Diablo, but if the expansion is installed, the player will have to kill Baal before the portal can be opened. A red portal will appear if the player has also killed the final boss on that difficulty. To create a portal to the secret level, combine Wirt's Leg and a Tome of Town Portal in the Horadric Cube while standing in the Rogue Encampment. The Secret Cow Level can be found in Diablo II (with or without the expansion).

Diablo 2 secret cow level not working