The Secret Cow Level
The Secret Cow Level (Moo Moo Farm) is a hidden area that can be accessed on any difficulty after completing the 
Eve of Destruction on that difficulty.
In the Secret Cow Level you will also encounter the Cow King. Killing the Cow King completes a hidden quests. In older versions of Diablo II, killing the Cow King would prevent you from opening the portal again in that difficulty.
The Cow King and other Hell Bovines have a chance to drop parts of the Cow King's Leather set:
How to Open The Cow Level
- Complete Eve of Destruction.
- Get
Wirt's Leg from Tristram on the same difficutly. - Transmute the Wirt's Leg and a
Tome of Town Portal in the 
Horadric Cube.
