The monster lived on one side of a narrow channel of water, thought to be the Strait of Messina, separating mainland Italy from Sicily. Her counterpart is Charybdis, a whirlpool-like sea monster that consumed water and ships alike. The two sides of the strait were within an arrow's range of each other—so close that sailors attempting to avoid Charybdis would pass too close to Scylla and vice versa. The idiom "between Scylla and Charybdis", refers to this situation of choosing between two evils.
There are several myths about her origin. Originally a nymph, some place her as a daughter of Phorcys and Ceto while others name Triton and Lamia as her parents. According to Hyginus she was beloved of the god Glaucus, who was himself desired by the sorceress Circe. She poisoned her rival's bath waters and turned her into a ravenous monster whilst others say she was pursued by Poseidon, whose wife Amphitrite transformed her.
Scylla's body was said to have consisted of twelve tentacle-resembling legs and a cat's tail with about four to six dog heads ringing up her waist. Sometimes Scylla is depicted as a large, angry woman with ravenous dogs instead of legs. Other times, it is described as a creature with many snake-like heads. Its habits are also described differently throughout Greek mythology.
In some myths, Scylla will only eat any random six sailors, while other myths say that she will relentlessly attack the ship by quickly devouring the sailors.