Scylla is one of the two guardians of the Straits of Messina. But her tragic story starts many years before she became a monster. Scylla, or Skylla, was once a beautiful sea nymph. A merman named Galcus fell in love with her but she rejected him. Galcus went to the witch, Circe, for help. The enchantress fell in love with Galcus herself and grew insanely jealous of Scylla. Circe prepared a potion that would change Scylla into a horrible monster and took it to the place where the nymph bathed. She poured the potion into the water and waited. Scylla eventually arrived and upon entering the water was changed into a six-headed monster with an insatiable hunger for red meat. Overjoyed, Circe returned to Galcus, certain that he would have her with Scylla out of the way. Galcus rejected Circe and then disappeared, forever.
Once Scylla realized she was a monster she tried to kill herself. But Posiedon took pity on her for she had once been one of his favorite nymphs and he placed her in the Straits where she was soon joined by another monster, Charybdis. In the episode Ulysseus and the Journey Home, she has a hundred tentacles and looks like a gigantic grey octupus with bulbous red eyes.
She has twelve mis-shapen feet, and six necks of the most prodigious length; and at the end of each neck she has a frightful head with three rows of teeth in each, all set very close together, so that they would crunch any one to death in a moment, and she sits deep within her shady cell thrusting out her heads and peering all round the rock, fishing for dolphins or dogfish or any larger monster that she can catch, of the thousands with which Amphitrite teems. No ship ever yet got past her without losing some men, for she shoots out all her heads at once, and carries off a man in each mouth.
Excerpt from The Odyssey, Book 12