List of characters in the Camp Half-Blood series
Scylla - One of the two monsters (the other being Charybdis) that live on either side of a narrow channel of water in The Sea of Monsters. Should a ship sail too close to her, she uses her six heads to pluck off crew members upon deck and devour them. According to Clarisse (daughter of Ares), if the crew hid below deck, Scylla would just pick up the whole boat and crush it.