Epithets are little nicknames or descriptive phrases that were used by Greek epic poets to tell their listeners about the characters and to preserve the meter of their poems.
In Book X, Circe is both a danger to Odysseus and his crew and, eventually, his lover who is devoted to him. Although she is a very important character in this book, epithets are not really used to describe her. In the first mention of her, we are told that Odysseus and his men came to the island
where Circe lives—a great and cunning goddess who is own sister to the magician Aeetes—for they are both children of the sun by Perse, who is daughter to Oceanus.
But this is not really an epithet because epithets are phrases that used along with the name of the person, not just descriptions that are tacked on after.
There is one epithet for Circe used in Book X. Odysseus says that he "was near the great house of the enchantress Circe." In this case "the enchantress Circe" can be seen as an epithet. There is no other description of Circe in Book X that could be seen as an epithet.