The boss Purgatory Judge is located in the top section of the Purgatory dungeon. Once within a range of around 500 around him he applies a debuff called Undead's Grasp : Cannot use skills and/or consumables, cannot move. HP reduced by 5% every 2 seconds. Lasts 10 seconds.
In Difficult and Nightmare modes, killing him a first time spawns a Bloodthirsty Judge immediately after the first one's death which is just a stronger version of it and able to use the same skills.