Diablo 4 developer hints that the search for the secret cow level is going in the right direction: 'If people have picked up any items from their cow campaigns, they should hang on to them'

pcgamer.com:

The search for the Diablo 4 cow level is much more than a cowspiracy (apologies) at this point. IGN spoke to the game's production developer Tiffany Wat, and associate game director Joe Piepiora at BlizzCon 2023 where hints were dropped, adding more fuel to the fire. It's not much to go on, but Wat may have hinted that players are heading in the right direction: «I will say, however, if people have picked up any items from their cow campaigns… cow slay-age campaign… they should hang on to them.»

The items Wat is likely referring to are the Bloody Wooden Shard, Musky Tome, and Metallic Fragment. You need to slay three sets of 666 cows, with your final kill being in distinct regions. Each set of cattle slaughter will end in a relic being dropped, which prompts a key to be dropped when used in Ken Bardu. The key is for Forlorn Hovel where you'll find 28 cows. Killing those cows drops some stamina potions (a reference to when you kill the Cow King in Diablo 2 and he drops stamina potions). This seems to be where the search has halted for now.

The cow level has a legacy of its own. Persistent rumours seemed to have pushed Blizzard to include one in Diablo 2 and 3. World of Warcraft even felt the effect of the powerful moovement with its own secret cow level back in 2017. With this in mind, the sheer will of the players looking for one in the latest game is not surprising.

There has been much development since Diablo general manager Rod Fergusson said the team had been «really focused on trying to keep it as grounded as possible» in an interview with Kinda Funny back in May, adding that " because of that, there is no secret level in Diablo 4 that people might be looking for." Since then though, hope for the bovine secret grows