Charybdis

Charybdis (Χάρυβδη in Ancient Greek) was once a beautiful Naiad, the daughter of Poseidon and a nymph. She takes form as a huge mouth that swallows great amounts of water and debris three times a day, creating massive whirlpools.

History
Charybdis was very loyal to her father in his endless feud with Zeus; it was she who rode the hungry tides after Poseidon had stirred up a storm, and led them onto the beaches, covering whole villages, submerging fields, drowning forests, claiming them for the sea. She won so much land for her father's kingdom that Zeus became enraged and changed her into a monster forever.

The myths say that Charybdis lay on one side of a blue, narrow channel of water. On the other side of the strait was Scylla, another sea monster. The two sides of the strait are within an arrow's range of each other, so close that sailors attempting to avoid Charybdis will pass too close to Scylla and vice-versa. The phrase "between Scylla and Charybdis" 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 of the other. "Between Scylla and Charybdis" is the origin of the phrase "between the rock and the whirlpool" (the rock upon which Scylla dwelt and the whirlpool of Charybdis) and may also be the genesis of the phrase "between a rock and a hard place". Odysseus had to pass by her and Scllya while trying to return home.

The Sea of Monsters
Clarisse takes her ironclad between Charybdis and Scylla to enter the Sea of Monsters, which is located in the Bermuda Triangle. She is a black coral reef with a tree on top and described by Percy Jackson as an "orthodontists' nightmare", a huge disgusting mouth with braces underneath. She is surrounded by the whirlpool she creates when she sucks up the sea and everything in her range. Clarisse chooses to take on Charybdis because she is an easier target for the cannons. Charybdis is always swallowing and spitting, unlike in the myths in which she only eats three times a day.

Trivia

 * Charybdis is technically Percy's half-sister, as they are both children of Poseidon.
 * Scylla and Charybdis may refer to the space between Sicilia and Italy, where sea currents are very strong.