Secret Cow Level
Accessible in all three difficulties, the Secret Cow Level is a hidden farming area built entirely around one joke that Blizzard made real. It offers dense monster packs, a simple layout with no branching paths, and a solid drop pool — making it a practical destination for mid-game and early endgame farming runs.
The portal is created inside the Horadric Cube. Combine Wirt's Leg with a Tome of Town Portal and hit Transmute while standing anywhere inside the Rogue Encampment.
The character opening the Cube must have killed Baal and completed Act 5 on the same difficulty you are trying to access. Characters that have not finished the final quest on that difficulty cannot open the portal.
Wirt's Leg is found on Wirt's corpse in Tristram. The entrance to Tristram is a red portal in Stony Field that spawns once the Cairn Stones have been activated using the Scroll of Inifuss. It appears reliably in every new game once that step is done. The minimap displays a yellow marker when you are close enough for it to register.
The Cow King occupies a fort-like structure that generates at a random position each game. Killing him triggers a short quest completion and produces a drop of 8 Stamina Potions — a deliberately anticlimactic reward that the community has long referred to as milking the Cow King.
The original Diablo shipped with a cow standing in Tristram that did nothing. It attracted enough attention that a persistent rumour spread claiming a hidden level could be unlocked by clicking it enough times. Blizzard addressed this directly by including "There is no cow level" as a cheat code in StarCraft. Two years later during Diablo 2's development, a screenshot surfaced on April 1st 1999 showing a player in combat with armed cattle. Blizzard gave no explanation. When the game launched, the level existed. The sound effects — all the Moo vocalisations — were recorded by members of the development team.