This page discusses how to reach the Secret Cow Level in Diablo 2 Resurrected.
After completing the game in Normal, Nightmare or Hell difficulty, you’ll be eligible to create a portal to the Secret Cow Level - one of the worst kept secrets from over two decades ago.
To do this, create a game in a difficulty you’ve completed, then head to the Stony Field. Your goal is Tristram, which can be found via the portal that Rakinishu guards (you’ll have to complete the quest The Search for Cain to create this portal, after which it’ll remain open on any subsequent games you create). Keep your minimap open while you search for this portal, as the yellow cross icon that indicates the portal will appear on your minimap from some distance away, allowing you to discover your destination easier.
Once you’ve located the portal to Tristram, head inside and make your way to the western-most corner, where you’ll find Wirt’s body. Loot it, and along with a cascade of gold you’ll find Wirt’s Leg, which is one of the components necessary to create the portal to the Secret Cow Level.
Return to town and buy a Tome of Town Portal (the quantity of portals within doesn’t matter) and put it inside your [Horadric Cube] along with Wirt’s Leg, and with these two being the only items in the cube, transmute them to create the portal. Make sure you’re standing somewhere convenient - summoning the portal near an NPC in town or on top of the stash can be a bother.
Once you pass through the portal you’ll encounter hordes of Hell Bovines, led by The Cow King. The most noteworthy feature of the Secret Cow Level is the sheer enemy density - it may not be the best spot to hunt for uniques, but runes, gems, charms and socketed items all drop fairly commonly here. You can also get the Cow King’s Leathers set, which only drops in the Secret Cow Level. It’s not a particularly useful set, but collectors and completionists may want to hunt it down, regardless.