Harmonia was born from an adulterous union that shook Olympus. Her mother was Aphrodite, the goddess of love and beauty, whose power over gods and mortals was absolute. Her father was Ares, the god of war, whose delight was in slaughter and whose affair with Aphrodite was the most scandalous in heaven. Hephaestus, the lame smith-god who was Aphrodite's husband, discovered the affair through the all-seeing sun-god Helios, and he trapped the lovers in a golden net so fine that it was invisible, then called the other gods to witness their shame. When the net was lifted, Aphrodite and Ares fled in opposite directions, but the child they had conceived was already growing in the goddess's womb. Harmonia was born from this humiliation, a child of love and war, of beauty and bloodshed, and her whole life would be a struggle to hold these opposites together.

She married Cadmus, the Phoenician prince who had come to Greece in search of his sister Europa, the girl whom Zeus had carried away in the form of a white bull. Cadmus had failed to find Europa, but he had slain the dragon that guarded the spring of Ares near the future site of Thebes, and he had sown the dragon's teeth in the earth, from which sprang the Spartoi, the Sown Men, who became the warrior aristocracy of the new city. For this murder of the dragon, which was sacred to Ares, Cadmus was sentenced to serve the war-god for eight years, and when his service was complete, the gods gave him Harmonia as a bride. The wedding was celebrated on the Cadmeia, the citadel of Thebes, and all the Olympians attended. Aphrodite gave her daughter a golden necklace forged by Hephaestus, a work of divine craftsmanship set with enchanted stones that caught the light like captured stars. It was the most beautiful piece of jewellery ever made, and it was cursed from the moment it was given.

The curse was subtle at first. Harmonia and Cadmus had five children: four daughters, Autonoe, Agave, Ino, and Semele, and one son, Polydorus. The daughters were beautiful and proud, and each of them married into the royal houses of Greece. But the necklace brought misfortune to every branch of the family. Autonoe married Aristaeus, a minor god of the countryside, and their son Actaeon was torn apart by his own hunting dogs after he saw the goddess Artemis bathing naked. Agave married Echion, one of the Spartoi, and their son Pentheus was torn limb from limb by the maenads of Dionysus, led by Agave herself in a frenzy of divine ecstasy. Ino married Athamas, the king of Orchomenus, and after she nursed the infant Dionysus, Hera, the queen of Olympus, drove her mad; she leaped into the sea with her son Melicertes in her arms and was transformed into the sea-goddess Leucothea. Semele, the youngest and most beautiful, was loved by Zeus himself, but when she demanded to see him in his true form, the thunderbolt of the king of the gods consumed her, and her unborn child, Dionysus, was sewn into Zeus's thigh to finish gestating.

The curse of the necklace did not stop with the daughters. It passed to their descendants, to the house of Labdacus and to Oedipus, the king who killed his father and married his mother, and to his sons Eteocles and Polyneices, who killed each other before the walls of Thebes. The necklace itself was passed from mother to daughter, from queen to queen, and each wearer met a violent end. Some accounts say that Harmonia eventually gave the necklace to her daughter Semele, and that it was buried with her when she died. Others say that it passed to Jocasta, the mother of Oedipus, and then to her daughter Antigone, and then to the wives of the Argive kings, each of whom suffered disaster. The curse was only broken when the necklace was dedicated in the temple of Athena at Thebes, or when it was thrown into the sea, or when the last descendant of Cadmus died childless.

In their old age, Cadmus and Harmonia left Thebes, driven out by the wars that their family had unleashed. They travelled to Illyria, in the western Balkans, where Cadmus had once ruled as a king, and there they prayed to the gods to be transformed into serpents, the form of the dragon that Cadmus had slain so many years before. The gods granted their prayer, and the two old people became snakes, their human memories fading into the cold wisdom of reptiles. Some accounts say that they were then transported to the Elysian Fields, the paradise of the blessed dead, where they lived forever in peace, their serpent bodies coiled together in the grass of the fortunate isles. Others say that they remained in Illyria, guarding a sacred grove, their transformation a punishment for the murder of the dragon and a reward for their long suffering.