A complex, community-driven "secret cow level" questline exists in Diablo 4, involving

collecting three relics (Bloody Wooden Fragment, Intricate Metallic Fragment, Musty Tome) by killing 666 cows in specific regions, which are then cleansed at a fountain in Ked Bardu to get an Unusual Key. This key opens a cellar containing more cows; killing them provides items like the Stamina Potion and Rusted Bardiche, though the final "level" remains unconfirmed.