Poe is the original master of suspense, and this short story, "The Cask of Amontillado," is evidence of that fact. The narrator talks to an invisible and unknowable audience (us), but there is someone else to whom he speaks. He says:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged....
These few lines establish one clear fact: the specific person to whom Montressor addresses these lines is someone who knows him well. We simply get to listen in on the story. The fact that he's telling this story fifty years after the fact suggests this is perhaps a letter or some other kind of written work, which means it could have been just about anyone who was at least somewhat familiar with the two main characters and this setting.
To answer your question, then, we can't be sure whom, exactly, Montressor is addressing. We know it was someone who knew him well and would, therefore, presumably not be as shocked as we are to hear about this outrageous episode in Montressor's life.
Readers are never told exactly who Montresor is telling this story to. We do know that he is telling this story many years after having killed Fortunato. The story ends with the narrator saying that it has been half of a century since the two men went down into that cellar.
I do not think that Montresor is specifically telling readers about that night though. I originally thought that Montresor is "speaking" to us (the reader) because he says "you." It's like he is directly addressing me or any other reader; however, I no longer think that is true. The full line that I am thinking of is the following line.
You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat.
Montresor can't be talking to me or some other nameless reader. He has to be talking to somebody that knows him. I don't know Montresor at all, so I can't know the nature of his soul. He must be talking to somebody that knows him well enough to know that Montresor is not somebody that announces his intentions to people. Readers essentially get to eavesdrop on this conversation between Montresor and the other person.
what is the purpose of telling the story ?