What are the roles of monsters in The Odyssey?

In a general sense, the various monsters confronted in the Odyssey serve as tests of Odysseus's cunning and resolve. Each one presents a unique type of obstacle to the adventurer, tempting or testing Odysseus in a different way, and they can each be understood to represent eternal human challenges, fears, and conflicts. Scylla and Charybdis, for instance, have become bywords for any choice between two equally destructive alternatives or extremes; they are literature's archetypal "rock and a hard place" or "no-win situation." The sirens and the sea nymph Calypso each embody variations of temptation; the sirens embody the false, cold allure of all things alien and malevolent; their beauty and song beckon Odysseus to surrender to the unknowable depths of the sea, as the worm beckons the fish.

Calypso, on the other hand, can be seen as the true and perfect temptation, the promise of arcane and eternal bliss which threatens to sway him from his course and from his human, temporal devotion to his wife, son, and kingdom. Calypso dramatizes the conflict of those qualities in us that desire struggle, discovery, change, and fulfillment, against those that desire ease, release, and inertia. In a sense, she offers the choice between the humane and the divine. The island of the lotus-eaters poses a similar kind of choice, between the striving and incertitude of reality and the addictive pleasures of opiate-induced slumber.

Of all the Odyssey's monsters, the cyclops Polyphemus is the hero's most direct and active antagonist, and he serves as an avatar of ruthlessness, barbarism, and destructive appetites. In these respects he becomes a sort of monstrous mirror of the suitors in Odysseus's palace. His role is primarily to test Odysseus's cunning, and to present a warped vision of the Odyssey's recurring theme of hospitality; like Circe, he is a predatory host, devouring Odysseus's men and violating sacred taboos which mandate generosity to one's guests--a theme which comes full circle when Odysseus slaughters the suitors in his own home, disguising his identity to fool them much as he did with Polyphemus.

Speaking broadly, the monsters Odysseus faces in his journey constitute a kind of gauntlet or series of trials, probing for specific weaknesses in his character. The fact that he prevails, alone among all his men, is an indication of the exceptional qualities that make him and epic hero and an exemplar of certain human limits.