Monster who guards the entrance of the Sea of Monsters. Has six heads on long necks which reach down and pluck sailors from their ship.
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In Greek mythology, Scylla was a monster that lived on one side of a narrow channel of water, opposite its counterpart Charybdis. 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.
Scylla was a horrible 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 cat's 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 Ceto (all of whom may be various names for the same goddess). Some sources, including Stesichorus, cite her parents as Triton and Lamia.
Traditionally the strait has been associated with the Strait of Messina between Italy and Sicily, but more recently this theory has been challenged, and the alternative location of Cape Skilla in northwest Greece has been suggested by Tim Severin.
The phrase "between Scylla and Charybdis" (popularly reworded "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.
Source(s): Percy Jackson and the Olympians - Ultimate Guide ;;Wikipedia