Who or what seems to save the narrator at the end of "The Pit and Pendulum"?
Your question is not actually as straightforward as it would first appear, because the richly symbolic nature of this tale opens its rather abrupt ending to a number of different interpretations. Ostensibly, the ending occurs when the French army invades Toledo and the narrator is saved from tumbling into the deadly pit at the very last moment by General Lasalle, who catches him just as he is about to fall. Note how the final paragraph of this incredible story describes this incident:
There was a discordant hum of human voices! There was a loud blast as of many trumpets! There was a harsh grating as of a thousand thunders! The fiery walls rushed back! An outstretched arm caught my own as I fell, fainting, into the abyss. It was that of General Lasalle. The French army had entered Toledo. The Inquisition was in the hands of its enemies.
However, if we examine the diction of this final paragraph carefully, it is clear that Poe could be suggesting the narrator's fall into the interestingly-named "abyss" represents man's fall into hell and therefore, that General Lasalle could symbolically represent Jesus Christ and his act of salvation for mankind. This is suggested by the reference to the sound of trumpets, which is richly symbolic of the end times as expressed in the Book of Revelation in the Bible.