A dangerous rock on the Italian coast opposite the whirpool Charybdis on the coast of Sicily, -- both personified in classical literature as ravenous monsters. The passage between them was formerly considered perilous; hence, the saying "Between Scylla and Charybdis," signifying a great peril on either hand.

(Greek mythology) a sea nymph transformed into a sea monster who lived on one side of a narrow strait; drowned and devoured sailors who tried to escape Charybdis (a whirlpool) on the other side of the strait

In Greek mythology, Scylla, or Skylla (Greek Σκύλλα) was a name shared by two characters, a female sea monster and a princess.