What is the cow level and how do I get there?
It's a secret level filled with mooing cows on par with end-game enemy difficulty. They drop unique, set, and magic items, and their King drops a rare item. It's more of a fun Easter egg than something to take advantage of.
Get Wirt's Leg on Wirt's corpse in the north while rescuing Cain from Tristam.
Finish the game, and return to Act 1 on the same difficulty.
In the Rogue Encampment, combine Wirt's Leg and a Town Portal Tome in the Horadric Cube.
Enter the red portal to Moo Moo Farm. If you want to return on the same difficulty in the future, don't kill the Cow King.
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To reach the Secret Cow Level, you must first have beaten Diablo. Once you've done this return to the Rogue Encampment.
Using the Horadric Cube, obtained in Act II, you will place a Tome of Town Portal and Wirt's Leg (obtained in Tristram), before selecting Transmute.
This will open a red portal within the Rogue Encampment that will take you to the cow level.
Within, you will find hundreds of vicious... cows... Armed with polearms. It's an excellent place to grind experience, especially if you're playing on a server. I was able to take my paladin from 50 to 99 in only a handful of runs. It's worth noting that if you kill the Cow King, you will no longer be able to create the portal. You can, however, still enter the portal if you're playing on a server in which someone else has created said portal.
The cow level can be reached by combining Wirt's Leg (found in old Tristram, where you rescued Cain) with a Tome of Town Portal (Purchasable from Akara) in the Hordric Cube. This must be done in the Act 1 Rogue Encampment, and can be done once per difficulty. Unless you manage to forgo killing the Cow King; in which case it can be done over and over until you do.
A portal will open up, to an area filled with cows standing upright on their hind legs. Some will be carrying weapons as well. This area has a much higher enemy density than most zones; be careful around the portal. If you die nearby, you may have too many enemies to get through to retrieve your corpse.
EDIT: Forgot to mention, you must have defeated Diablo at this difficulty already.
Source: http://www.diablowiki.com/Horadric_Cube_Recipes_%28Diablo_II%29#Quest.2FOther