Scylla is the daughter of Keto and Phorcys and one of the two monsters (the other being Charybdis) that lived on either side of a narrow channel of water. 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" (or between a rock and a hard place) has come to mean being in a state where one is between two dangers and moving away from one will cause you to be in danger from the other.
Scylla was a grotesque sea monster, with four eyes, six long necks equipped with grisly heads, each of which contained three rows of sharp teeth. Her body consisted of twelve tentacle-like legs and a tail and with four to six dog-heads ringing her waist. She was one of the children of Phorcys and either Hecate, Crataeis, Lamia or Keto (where Scylla would also be known as one of the Phorcydes). Some sources, including Stesichorus, cite her parents as Triton and Lamia. Hesoid calls her the daughter of Hecate and Apollo.
According to Ovid, Scylla was once a beautiful nymph. The fisherman-turned-sea-god Glaucus fell madly in love with her, but she fled from him onto land, where he could not follow. Despair filled his heart and he went to the sorceress Circe to ask for a love potion to melt Scylla's heart. As he told his tale of love about Scylla to Circe, she herself fell in love with him. She wooed him with her sweetest words and looks, but the sea-god would have none of her. Circe was furious, but with Scylla and not with Glaucus. She prepared a vial of very powerful poison and poured it in the pool where Scylla bathed.
As soon as the nymph entered the water, she was transformed into a frightful monster with twelve feet and six heads, each with three rows of teeth. Angry, growling wolf heads grew from her waist, and she tried to brush them off. She stood there in utter misery, unable to move, loathing and destroying everything that came into her reach, a peril to all sailors who passed near her. Whenever a ship passed, each of her heads would seize one of the crew.
Clarisse La Rue sails her ironclad between Scylla and Charybdis to enter the sea, which is actually located in the current day Bermuda Triangle. Clarisse chooses to sail closer to Charybdis, but during the ensuing fight, the ship was pushed too close to Scylla, and she manages to snap up some of the dead Confederate sailors that manned the boat. Scylla grabs hold of Percy's backpack but Percy manages to poke her in the eye, before the CSS Birmingham explodes.