To get to the Secret Cow Level, you must have beaten the game on that difficulty, then go to the Rogue Encampment, take Wirt's Leg and a Tome of Town Portal into the Horadric Cube, transmute it and a portal to the Cow Level appears, there are hordes or cows here, be warned, once you kill the Cow King, you cannot go there again on that difficulty so you can go there 3 times, once on each difficulty if you kill him. However if you do not kill the Cow King, you can keep going there and get lots of items and gold.
Increase Difficulty In-Game
To do this, you must press the ENTER key as soon as you join the game, and type in ''/players X'', replacing X with a number from 1 to 8. For example, entering ''/players 8'' will make the game as hard as it would be if there were 8 players in it, which will give way more EXP and better drops, but the monsters will be much harder. Note: This does not work with the Lord of Destruction expansion.
