The Secret Cow Level
The Secret Cow Level (internally the Moo Moo Farm) is a hidden area full of Hell Bovines. Its portal can be opened on a difficulty once you have completed 
Eve of Destruction on that difficulty. The area is level 28 in Normal, 64 in Nightmare and 81 in Hell.
Hell BovineThe Secret Cow LevelHow to Open The Cow Level
- Complete Eve of Destruction on the difficulty you want to open the portal on.
- Pick up
Wirt's Leg from Wirt's corpse in Tristram. - Transmute the Wirt's Leg and a
Tome of Town Portal in the 
Horadric Cube in the Rogue Encampment.
The Cow King
The Cow King is a super unique Hell Bovine that roams the level with six minions. He always spawns Magic Resistant and Lightning Enchanted, which makes him immune to lightning in Hell. The first time a character kills him on a difficulty, he also drops 8 Stamina Potions.
Killing him completes a hidden quest. In the original Diablo II, a character that had killed him could no longer open the portal on that difficulty; Diablo II: Resurrected dropped that rule, so you can kill him and keep farming cows.
Cow King's Leathers
The Cow King's Leathers set is named after him, but neither he nor the Hell Bovines are more likely to drop it: like any set item, its parts drop from any monster whose level is high enough: 20 for the Hide and Hooves, 34 for the Horns (a War Hat).
