Echidna was born in the earliest days of the cosmos, before the Olympian gods had secured their throne, when the world was still populated by primordial beings of immense and terrible power. Her parentage is disputed among the ancient sources. Hesiod, the poet who wrote the Theogony, the great genealogy of the gods, names her as the daughter of Ceto and Phorcys, the ancient sea deities whose own children included the Gorgons, the Graeae, and the dragon Ladon. Ceto was the goddess of the sea's monstrous depths, and Phorcys was the old man of the sea who dwelled in a cave at the ocean's bottom, a creature half man and half fish with crab-claw forelegs. From this union, Echidna inherited the darkness of the deep and the hunger of the abyss. But the mythographer Apollodorus, writing centuries later, gives her a different and even more elemental origin: she was the daughter of Gaia, the primordial earth goddess who is the living world itself, and Tartarus, the pit beneath the earth where the wicked suffer eternal torment. In this version, Echidna is not a sea creature at all but a being of the underworld, born from the earth's darkest womb.

Whatever her parentage, every source agrees on her form. She was a drakaina, a she-dragon, with the head and torso of a beautiful woman and the body of a huge speckled serpent. Her face was lovely, her eyes dark and glancing, but below the waist she was all monster: a coiling serpent's tail, sometimes one and sometimes many, with scales that gleamed in the darkness of her cave. Some later writers, including the comedian Aristophanes, gave her a hundred serpent heads to match her husband Typhon's hundred fire-breathing heads, but the earliest accounts kept her form simpler and more terrible: a woman above, a viper below, a creature that could lure the unwary with beauty and then strike with poison. She lived in a cave deep beneath a hollow rock in the country of the Arimi, a place so remote that Hesiod says it was far from both the deathless gods and mortal men, a hiding place appointed by the gods themselves where she could dwell in isolation and breed her terrors.

Her mate was Typhon, also called Typhoeus, the most monstrous being in Greek mythology. Typhon was a giant of immense size, born from Gaia's anger at Zeus for destroying the giants, her earlier children. In some accounts, Hera, the wife of Zeus, prayed to Gaia and Uranus for a weapon against her husband and was given two eggs that produced Typhon. In others, Gaia conceived him directly with Tartarus. Typhon had a hundred serpent heads for fingers, eyes that blazed with fire, and a voice that roared like thunder. He challenged Zeus for the throne of Olympus and came closer than any other being to overthrowing the king of the gods, tearing out Zeus's sinews and imprisoning him in a cave in Cilicia before the god recovered and buried Typhon beneath Mount Etna in Sicily, where his struggles still cause the volcano to erupt. But before that final battle, Typhon found Echidna in her cave, and the two monsters were joined in love, a union that the poet Hesiod describes with surprising tenderness as the source of the most terrible brood the world has ever known.

Their first child was Orthrus, the two-headed hound who guarded the cattle of the triple-bodied giant Geryon on the island of Erytheia at the western edge of the world. Heracles, the greatest hero of Greece, killed Orthrus during his tenth labour when he came to steal the cattle. Next came Cerberus, the three-headed dog - or in Hesiod's account, the fifty-headed hound of Hades - who guards the entrance to the underworld, preventing the dead from escaping and the living from entering. Heracles captured him alive during his twelfth and final labour, dragging the beast up to the surface world and back again. Third was the Lernaean Hydra, the many-headed water serpent that haunted the swamps of Lerna in the Argolid, a creature so venomous that its breath could kill a man at a distance, and so regenerative that for every head Heracles cut off with his sword, two more sprang from the stump. Heracles finally defeated it with fire, searing the necks so the heads could not regrow. Fourth was the Chimera, a creature of Lycia in Asia Minor, with the forepart of a lion, the middle of a goat, and the hindpart of a serpent, breathing fire from all three mouths. The hero Bellerophon, riding the winged horse Pegasus, killed the Chimera by driving a lead-tipped spear down its throat, melting the lead in its fiery belly and choking it from within.

Later writers added more children to Echidna's brood, expanding her monstrous family until it seemed that every creature that ever tested a Greek hero had sprung from her cave. The mythographer Acusilaus, writing in the sixth century BCE, added the Caucasian eagle, the giant bird that Zeus sent to tear out the liver of Prometheus, the Titan who had stolen fire for mankind, every day for eternity, the liver regenerating each night so the torture could begin again. Heracles shot the eagle with an arrow during his travels. The mythographer Pherecydes added Ladon, the hundred-headed serpent who guarded the golden apples in the garden of the Hesperides, the nymphs who tended the tree at the far western edge of the world. Heracles killed Ladon with arrows poisoned with the blood of the Hydra, another of Echidna's children. The poet Lasus of Hermione added the Sphinx, the winged creature with a woman's face and a lion's body who devoured those who could not answer her riddle, until Oedipus solved it and caused her to throw herself from a cliff. Apollodorus added the Crommyonian Sow, the monstrous pig that terrorised the region between Megara and Corinth until Theseus killed it on his way to Athens. Hyginus, the Roman mythographer, added the Gorgon, the first of the three sisters whose faces turned men to stone; Scylla, the six-headed sea monster who devoured sailors in the strait between Italy and Sicily; and the Colchian dragon that guarded the Golden Fleece in the kingdom of Aeetes, which Medea the witch put to sleep so that Jason could steal the treasure. Some even added the Nemean lion, whose hide was impervious to weapons and whom Heracles strangled with his bare hands, and the Harpies, the filthy winged creatures with women's faces who snatched food and cursed travellers.

Echidna's own fate is told differently in different sources. Hesiod, who first gave her form and family, says that she was immortal and ageless, and that she continued to live in her cave in Arima, carrying off passers-by and devouring them, a permanent terror in the landscape of the world. But Apollodorus tells a different story. After Typhon was defeated and buried beneath Mount Etna, Echidna survived and continued to prey upon travellers. Hera, the queen of the gods, recognising the threat that this mother of monsters posed, sent her loyal servant Argus Panoptes, the hundred-eyed giant who never slept, to destroy her. Argus crept into Echidna's cave while she slept and killed her, ending the reign of the mother of monsters. Whether she died by the giant's hand or lived on forever in her cave, Echidna's legacy was secure: every monster that a Greek hero ever faced, from the gates of Hades to the gardens of the Hesperides, from the swamps of Lerna to the cliffs of Thebes, could trace its bloodline back to the woman with the beautiful face and the serpent's tail who lived in the cave of Arima.