To open the Secret Cow Level in Diablo II, you need Wirt's Leg (found in Tristram in Act 1), a full Tome of Town Portal, and must have killed the final boss (Diablo in Classic, Baal in Lord of Destruction & Resurrected) on that specific difficulty, then combine them in the Horadric Cube in the Rogue Encampment. A key restriction in LoD (lifted in D2R) prevents a player from opening it again on a difficulty if they've killed the Cow King there, but you can still join others.
The Secret Cow Level can be found in Diablo II (with or without the expansion). To create a portal to the secret level, combine Wirt's Leg and a Tome of Town Portal in the Horadric Cube while standing in the Rogue Encampment. A red portal will appear if the player has also killed the final boss on that difficulty.
Mechanics
In the original version of the game, you could access the Cow Level multiple times; however, Blizzard has changed this in subsequent updates to the game, so that now the level can only be accessed once per difficulty level, so a character can only get there a total of three times. UNLESS you do not kill the Cow King.
The cow level originated as a hoax about a secret level perpetrated by players of 1997's Diablo, and has been described as one of the most famous and well-known urban legends related to video gaming by publications.
The Cow Level is a level 81 zone in Hell, so it does not drop the best items (level 85+), but it does drop a high number of items, making cows also ideal for farming runes.
After Akara has translated the runes, return to Stony Field and locate the Cairn Stones, which are guarded by a Super Unique Carver named Rakanishu. Using the Scroll of Inifuss as a guide, touch each of the Cairn Stones in the correct order to open a portal directly to Tristram.
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 Javazon, aka Cow Blaster 9000, is among the strongest cow-farmers in the game. Even on a low budget, she can get through The Secret Cow Level quickly. Make sure to "heard up" a couple of packs of Hell Bovines to make full use of the Lightning Fury and Pierce interaction.
Cow King's Leathers is an item set in Diablo II: Lord of Destruction. This set can only be collected in the Secret Cow Level, by killing any cows, not just the Cow King. It is a mid-low level set, with solid stats for its level that nevertheless pales in comparison to high level gear.
A cow follows a player who holds wheat but stops following if separated from the player by at least six blocks. Adult cows enter love mode and pair off to create calves after they are given wheat, granting the player one to seven experience orbs.
Area level 85 zones
In Diablo II, the Cow King is the Super Unique boss of The Secret Cow Level. It looks just like any other Hell Bovine: there aren't any visual clues to identify it, except highlighting.
Canada's grading system categorizes cull cow carcasses into grades D1, 2, 3 and 4 based on the following: D1. excellent muscling, with less than 15 mm of firm, white or amber coloured backfat. D2. medium to excellent muscling, with less than 15 mm of white to yellow backfat.
Lying down is as an important part of the cow's day as it is ours. But they do it very differently to us. Cows lie down for about 8-12 hours per day, but unlike us, where we usually go to bed for one long block of time, they have a continuous pattern of eat-lie-repeat, with each lying bout being between 1-2 hours.
The large size of these mammals can mean that it takes longer to kill them when hunting. A long scope rifle, or a Bow with Improved Arrows, are considered to be the best weapons to use whilst hunting. Their hide can be sold to traders, and their prime beef is a good source of food."