Scylla was born to Crataeis, the goddess of sea dangers whose name means mighty or rocky, and perhaps to Phorkys, the ancient god of the deep sea who fathered many monsters. Some say her mother was Hecate, the goddess of witchcraft and crossroads who carries twin torches and walks between the worlds of the living and the dead. Others claim she was the daughter of Lamia, the child-devouring monster, or even of Typhon and Echidna, the most terrible monsters in all Greek mythology, whose offspring included the Hydra and the Chimera. Whatever her parentage, Scylla was a creature of the sea from birth, dwelling in a cavernous lair on the Italian mainland side of the narrow strait that separates Sicily from the toe of Italy. The Greeks called the strait the Pillars of the Sea, and they knew that any ship passing through must run the gauntlet between two terrors. On the Sicilian side lay Charybdis, the whirlpool that swallowed and vomited the sea three times a day. On the Italian side lay Scylla, who never slept and never stopped hungering.

The most famous encounter with Scylla comes in Homer's Odyssey, the epic poem that tells of the hero Odysseus and his ten-year voyage home from the Trojan War. After escaping the island of the witch Circe, the goddess of magic who had turned his men into pigs and then restored them, Odysseus received a warning. Circe told him that he must pass through the strait, and she gave him a terrible choice. Sail close to Charybdis, and the whirlpool might swallow the entire ship and all its crew. Sail close to Scylla, and she would snatch six men from the deck, but the rest would survive. Circe advised him to sail nearer to Scylla, to accept the loss of six rather than risk the loss of all. She also told him to invoke Scylla's mother, Crataeis, to prevent the monster from striking more than once. Odysseus, following this counsel, ordered his men to row hard and not to look up at the cliffs. But when they were distracted by the churning of Charybdis on the other side, Scylla struck. Six heads darted out from her cave, each on a long snaky neck, and seized six of Odysseus's best men from the deck. The poet describes them writhing and screaming, their arms reaching toward their companions in a last desperate appeal, as Scylla dragged them to her cave and devoured them alive at the entrance. Odysseus could only watch, frozen by horror, as his men were eaten in a place where no help could reach them.

But Scylla was not always a monster. In the later tradition, most fully told by the Roman poet Ovid in his Metamorphoses, she was once a beautiful nymph who bathed in a sea pool near the rocks of Zancle. The sea god Glaucus, who had once been a mortal fisherman from Boeotia before eating a magic herb that transformed him into a fish-tailed deity, saw Scylla and fell in love with her. He pursued her, declaring his devotion with words that spoke of his transformation and his divine nature, but Scylla fled in terror at his strange appearance. Glaucus, desperate, went to the sorceress Circe and begged for a love potion that would win Scylla's heart. Circe, however, desired Glaucus for herself. When he refused her advances, she turned her jealousy into vengeance. She went to the pool where Scylla bathed and poured a baleful potion into the water. When Scylla entered the pool, her lower body erupted into six dog heads on snaky necks, and her waist was girdled with the baying heads of hounds. She tried to flee from herself, but the dogs followed wherever she went, attached to her own body. In despair, she took refuge in the cave beneath the fortress of Scylla in Calabria, where she remained, devouring any sailors who came within reach, a beautiful face above and a monstrous body below.

Another version of her transformation comes from the Byzantine writer John Tzetzes and the commentator Servius, who recorded that Scylla was beloved by Poseidon, the god of the sea who ruled the oceans with his three-pronged trident. Amphitrite, Poseidon's wife and queen of the sea, was consumed with jealousy. She poisoned the spring where Scylla bathed, and the nymph was transformed into the six-headed monster. In yet another tale, recorded by the mythographer Hyginus, Scylla was simply a beautiful maiden who attracted the attention of many suitors, and Circe poisoned her out of spite. Whatever the cause, the result was the same: a creature that was half woman and half ravening beast, forever hungry and forever alone in her cave. Some stories say that Heracles, the hero forced to complete twelve impossible labors, encountered Scylla during his journey and killed her, but her father Phorkys restored her to life with flaming torches. This death and rebirth only added to her terror, for it meant she could not be killed, only endured.

In later ages, Scylla became more than a monster. She became a symbol of the impossible choice, the danger that cannot be avoided without running into another. The phrase between Scylla and Charybdis entered the languages of the world, evolving into between a rock and a hard place in English, though the original image is more vivid: two monsters so close that an arrow shot from one side could reach the other. The strait itself became a real place, the Strait of Messina, where sailors for centuries reported strange currents and jagged rocks that could tear a ship apart. The town of Scilla in Calabria still stands above the cave where she was said to dwell, and the rocks beneath the fortress are still called the Scyllaean rocks. In Roman times, Virgil sent Aeneas, the Trojan hero who fled the burning city to found Rome, past her lair on his way to Italy, and the poet warned that it was better to sail far around Sicily than to risk her gaze. The Byzantine encyclopedia Suda described her as having a beautiful face above the eyes, with six dog heads on each side and a serpent body below, a description that mixed beauty and horror in a way that still disturbs. Scylla is the eternal warning that the sea is not empty, that the rocks have teeth, and that every journey requires a sacrifice.