The narrator has escaped from the pendulum, but he sees that at the moment of his escape, the pendulum stops swinging and is drawn back up to the ceiling. He knows that he is being watched now. He begins to notice the red-hot smell of the burning walls, and the movement of the walls as they change the shape of the chamber. He is afraid of the pit, but to embrace the walls will also kill him.
If the narrator tries to push back against the walls or run into them, he will die, but he won’t die by falling into a dark pit.
If the narrator allows the walls to push him toward the pit, ultimately he will have no other choice but to fall over the side.