What hint does Homer give that even Poseidon must bow to the will of the Fates?
In Homer's Odyssey, Odysseus' primary divine enemy is the sea god Poseidon. In the early lines of the poem, Homer tells us that Odysseus will return home in spite of Poseidon's wrath, which might indicate that Poseidon cannot overcome what must happen:
Yet all the gods pitied him, except Posiedon, who continued his relentless anger against godlike Odysseus, until he reached his own land at last. (A.S. Kline translation)
Later in Odyssey 1, Zeus says the reason that Odysseus has suffered so much since his departure from Troy is because Odysseus incurred the wrath of Poseidon by blinding one of Poseidon's sons, the Cyclops Polyphemus. Zeus, however, notes that he wants to help Odysseus get back home and indicates that if all the other gods band together, then Poseidon will not be able to oppose them:
"Come, let all here plan how he might come home: then Poseidon will relent, since he’ll not be able to contend, alone, against all the deathless gods together." (A.S. Kline translation)
In Odyssey 11, Odysseus tells of his encounter with spirits from the underword. During that adventure, the spirit of the prophet Teiresias told Odysseus that should he eventually return to his native land that he would have to make sacrifice to Poseidon to appease the god's wrath. Teiresias' words give a strong indication that if Odysseus follows a certain course of action, then Poseidon will not be able to prevent Odysseus from reaching his home.
We must keep in mind, though, that what the Homeric gods want and what the Fates decree is not necessarily the same thing. In Iliad 16, for example, Zeus did not want Hector to kill his son Sarpedon, but Hera reminded Zeus that he should not go against what the Fates had in store for Sarpedon. Zeus relented and allowed Sarpedon to die in accordance with what the Fates had in store for him.
In the Odyssey, Zeus seems to indicate that Odysseus' homecoming is in accordance with the plan of the Fates when he says:
"This is the way he is fated (Greek: moir') to see his people again, his vaulted palace, and native isle." (A.S. Kline translation)
Here, the Greek noun moira is a word that Homer uses to denote "destiny" or "fate." Thus, this would seem to indicate that the Fates (Greek: Moirai) have sanctioned Odysseus' return home.
So, there are several places in the epic where it is clearly indicated that Odysseus is going to return home no matter what what Poseidon does.