A key part of Diablo 4 Season 9's endgame is farming Uniques and Mythics from the Lair Boss ladder, but not all of these ...

Betrayer’s Husk is a Lair Boss Key used exclusively to open Belial’s Hoard after defeating him in the Palace of the Deceiver.

Players can get Betrayer's Husks by beating Belial, but only if he appears after you open any of the Boss Hoards in the game.