The devil’s presence is toxic; the host may show signs of rapid aging, blackened veins, or "cold" fire.
Furthermore, Silas Vane (the human remnant) fights back. Vane hides "exit doors" inside the nightmares. These appear as out-of-place doors—a bright red door in a grey hallway, or a door painted with a white eye. Opening this door ends the visitation instantly. This is why the Nightmaretaker moves slowly. He is fighting his own possessed body. the nightmaretaker: the man possessed by the devil guide
If you encounter the entity directly and haven't been spotted, looking away and moving slowly can sometimes prevent a chase sequence. The devil’s presence is toxic; the host may