The Cow King is the Super Unique boss of The Secret Cow Level in Diablo II. It looks just like any other Hell Bovine: there aren't any visual clues to identify it, except highlighting. However, the Cow King always spawns inside the fenced enclosure located in the secret cow level, so players who wish to avoid killing him can just avoid the enclosure altogether.
Besides these two attributes that always appear, the Cow King gains one additional attribute (random modifier) on Nightmare and two on Hell. One of the most annoying attribute that it can get is Aura Enchanted: a Conviction aura Cow King coupled with its natural lightning enchanted can make fighting the Cow King a very hazardous task. It can also hold a range of immunities which can be quite annoying for pure-elemental sorceresses and other element-dependent characters. The Cow King is the only creature in the entirety of Diablo II to drop "The Cow King's Leathers".
Once he is killed, the Portal to the Secret Cow Level cannot be created again in the same difficulty. Unless you have one of the older versions of Diablo II, in the first edition you can open it an infinite number of times on the same difficulty. So, that makes the Cow King's Leathers the hardest set to complete in one game because there are 3 set pieces and the player gets only 3 chances to kill the King - one in each Difficulty. This results in a popular tactic amongst those that wish to run the level to avoid slaying him at all costs.
The Cow King is known more for his infamy among the rumor mills than for his drops. Ever since his discovery as an easter egg, he has been associated with various other fake hidden eggs. He was supposed to:
- Be able to drop the Bovine Armor unique item from Diablo: Hellfire
- Be able to grant access to a hidden Hireling - the Angel in Act IV
- Be related to Lester the Farmer from Diablo: Hellfire
- A NPC called "Ghost of the Cow King" appears in the old tristram road in Diablo III, next to the portal to the Pony Level (the game's Cow Level), the ghost refers to the "fact" that there is "no" cow level after the player character breaks the fourth wall by refering to said level about cows. The denial reflects Blizzard's perpetual stance that "there is no cow level".