How do they try and kill him in the end?

"The Pit and the Pendulum" is a story of suspense and terror. The narrator is subject to extended torture as he lies imprisoned beneath a suspended "scimitar," or blade, that swings like a pendulum ever closer to him. At the end of the story, however, after the narrator has escaped this doom, he finds himself immediately faced with another. More than anything else, he does not want to be pushed into the pit, or "abyss," he knows is in the room with him. In order to try and force him into the pit, the walls of the room become blazingly hot, and the shape of the room begins to change. It becomes "lozenge-shaped" and the narrator is forced to move further and further away from the heat and towards the pit until "there was no longer a square inch of foothold on the floor of my prison."
In the end, of course, the narrator does not fall to his death in the pit, for as he begins to fall his arm is caught at the last moment by his savior, General Lasalle.