Shadow Lurkers are the invisible keepers of Tormented Gates. They are only visible to those who can see into the Shadow Realm.
In order to break their grasp War must complete a challenge, typically killing demons in a specific fashion, usually within a time limit. They take the appearance of an over-sized beholder gate eye, but can be killed with only a few slashes from Chaoseater, making them one of the weakest foes in game.
They have no physical attacks, but they serve a similar purpose to locked doors as they are just obstacles. While a challenge is active, they are invulnerable and cannot be bypassed, but when the challenge is complete, they can be killed easily, and the portal behind them can be accessed.