Why does Montresor tell Fortunato about the amontillado?
Ah, the much-discussed and possibly non-existent amontillado. Montresor's untimate goal, of course, is to avenge himself upon his "friend" Fortunato for the unnamed "thousand injuries" and "insult" the latter has inflicted upon the former. Knowing his friend's vanity, Montresor lures Fortunato to his house by claiming he has come upon a pipe (a large quantity) of amontillado, a rare dessert wine (and during carnival!)
Montresor further suggests that he doubts it is genuine, though of course his dear friend Fortunato, a connoiseur of fine wines, would know at once. Without hesitation, Fortunato takes the bait and insists on checking the wine to be sure his dear friend Montresor has not been cheated.
Ultimately, Fortunato is lured to his death, and the reader will never know if there was, in fact, any wine at all in the cellar.