The Secret Cow Level
The Secret Cow Level 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 Tome of Town Portal in the Horadric Cube. A red portal will appear in the Act I starting area.
You can only create this portal when playing at a difficulty level you have already completed. For example, you must first beat the game (through Act IV) 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 cows. A rather special cow, named "The Cow King", always appears in the level. If someone kills the Cow King for the first time (virgin kill) 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 immuned from getting "the kill.
