How do you know that Helios's cattle will be slain? the odyssey

Because Tiresias says so in Book 11.

Explanation:

In Book 11 of the epic poem "The Odyssey", the main character Odysseus descends into the Underworld. There, he meets Tiresias, a legendary prophet, who gives him a warning. Tiresias tells Odysseus that if he and his men wish to come home soon and unharmed, they must learn to curb their desires. Tiresias knows, however, that Odysseus' men won't be able to do so. For that reason, he also warns Odysseus about Helio's cattle. The cattle are sacred, special, and should not be killed and eaten like regular animals. If his men end up killing the cattle, they will be murdered; Odysseus will have to return home alone, late, and as a broken man. That prediction is exactly what happens.

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