If you didn't read the book, I'll give you a short summary of what you need to know from Book 12 to answer this question.
PART ONE
In The Odyssey, Book 12, Circe advises Ulysses on how they can be safe from the lure of the Sirens on their journey back home. She also tells how to avoid the clashing rocks, including how they should escape the monster, Scylla, and the whirlpool known as Charybdis.
PART TWO
Ulysses tries to avoid stopping on the island of Thrinacia. He does this because he fears his men will harm the cattle that belongs to the sun god. (He had been warned against visiting the island while he was in the underworld.) His men end up killing the cattle anyway. They do this in an act of impiety against the gods. This shows much greed and a high lack of self-restraint. They completely ignore their promise to Ulysses and enrage Sol, the sun god, who implores the gods to punish Ulysses and his men.
So, this proves that the complete sentence should be...
<em>In Book 12, Ulysses is warned by</em> Circe <em>about the impending dangers of Scylla and Charybdis. She tells him not to harm the cattle of the sun god,</em> Sol.
