The Secret Cow Level

"Citizens of Sanctuary, please remain calm. There is no cow level."

- The Cow King

The Secret Cow Level, also known as the Moo Moo Farm, can be found in Diablo II (with or without the expansion Diablo II: Lord of Destruction). To create a portal to the secret level, combine Wirt's Leg and a Tome of Town Portal in the Horadric Cube. A red portal will appear in the Act I starting area.Players can only create this portal when playing at a difficulty level they have already completed. For example, one must first beat the game (through Act IV in the original game or Act V in the expansion) on Normal difficulty before you can create the Secret Cow Level in a game of Normal difficulty.

The Secret Cow Level is a small area, full of Hell Bovines: bipedal cattle with polearms. A rather special cow, named "The Cow King", always appears in the level. If someone kills the Cow King for the first time on the current difficulty, all players within the Cow Level, partied or not, will get "the kill" (he/she will be unable to create another portal to the Secret Cow Level on the same difficulty). Furthermore, if someone already has killed the Cow King on the current difficulty in a previous game and decides to kill the King in another game, everyone within the Cow Level will be ineligible to get "the kill."

The cow level was the most popular place to level up in, 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. The experience and item gains possible in the level were nerfed in, and the level returned to its intended purpose as balanced and not a place to endlessly exploit for massive experience gain, thus avoiding any grinding.

Trivia

 * There were many rumors that there was a "Secret Cow Level" in the original Diablo. Before this level was in fact created for Diablo II, the Jersey's Jersey quest for Diablo: Hellfire was created to reference the rumor. Nowadays, Blizzard still jokingly state that "there is no Cow Level".
 * One of the tooltips on Diablo III loading screen states: "Cow level is a lie". This appears to be a reference to the famous Portal game series line "The cake is a lie".
 * A free web enhancement called To Hell and Back, available at the Wizards of the Coast site, contains the Secret Cow Level with statistics for the enemy cows.
 * One of the cheat codes on StarCraft, "There Is No Cow Level", was made to confirm there was no cow level made. Later on it was still added.
 * There is a leather chest armor item in the World of Warcraft called Cow King's Hide, an obvious reference to the secret level.
 * In the Warcraft III: Frozen Throne chapter Curse of the Blood Elves, there is a secret level that could be accessed to unlock Blizzard's version of a Tower Defense. In the course of unlocking, many sheep are involved, which brought up the 'Secret Sheep level', a reference to the Secret Cow Level.
 * In Matthew Reilly's novel, The Six Sacred Stones, a pair of twins (who aid the main characters) frequently sport shirts that say "I have seen the Cow level!" and subsequently "There is no cow level!"
 * One item set, the Cow King's Leathers, can only be found in the Secret Cow Level.
 * A variation of this level, called "No Level Name" can be made if the portal maker creates the level at the boundary of the Rogue Encampment and the Blood Moor.
 * The Hell Bovines' first victim was Wirt, which could explain why his peg-leg is the key to 'The Secret Cow Level'.
 * Upon entering the "Secret Cow level" portal "Entering moo moo farm" is shown.
 * In Warcraft III, you can find "Wirt's Other Leg" which, of course, is a reference to "Wirt's Leg" and in turn is a reference to Wirt in the first game.
 * In Starcraft II, it is possible to join the "Cow Level" chat room, although not by ordinary means.
 * In World of Warcraft, you can obtain an item called "Wirt's Third Leg".
 * A "secret cow level" has been discovered for Diablo III, accessed though the help of the Ghost of the Cow King.
 * In Reaper of Souls there is a Cow Level in the Act V Nephalem Rift in Adventure Mode.
 * The Cow Level logo has been used for promotion of an unannounced Blizzard project.
 * A "Not So Secret Cow Farm" can be found in Goat Simulator, as accessed from a town called "Twistram." Bi-pedal cows attack the goat by shooting lasers from their udders.
 * In Marvel Heroes 2015, there is a "Classified Bovine Sector" accessible via special portal, that leads to an updated version of the level, with the boss cow being "High Commander Brevik." There is also a new, winter cow level, named "Bovenheim" that is much bigger and has a new boss, "All-Father Brevik".
 * For a short time in January 2017 to celebrate Diablo's 20th Anniversary, World of Warcraft players who completed Legion dungeons would spawn a naughty Treasure Goblin. Defeating him would reward some additional loot and open a portal to a place that has long since only been rumored to exist—the infamous Cow Level. Killing the Cow King will reward a unique toy and a Feat of Strength. Slaughter enough goblins and you'll be able to complete a Tome of Town Portal toy or collect a 28-slot Horadric Satchel.
 * In the World of Warcraft expansion Battle for Azeroth there is an achievement Lactose Intolerant  that can be earned while fighting a boss that will allow players to briefly transport to a secret cow level.  Slaughter enough cows and you'll earn the achievement which will yield progress towards a special mount.