Thetis was a sea goddess of the ancient Greek world, the most celebrated of the fifty Nereids and the mother of the hero Achilles. Daughter of the shape-shifting sea-god Nereus and the Oceanid Doris, she moved between the depths of the sea and the halls of Olympus with equal ease. Homer gives her a role far larger than her modest rank as a nymph would suggest, for gods and heroes alike turned to her in their moments of greatest need. Readers should not confuse Thetis, a Nereid of the younger generation, with Tethys, the elder Titaness who was one of the primordial powers of the sea.
Leader of the Nereids
The Nereids were the fifty daughters of Nereus, the Old Man of the Sea, and each embodied some gentle aspect of the water: the swell of the waves, the foam on the shore, the safe harbour that welcomed sailors home. Among these sisters Thetis stood first in beauty and in honour. She dwelt in a silver cave beneath the waves alongside her father, and from there she rose to the surface or climbed to Olympus when duty called. Like Nereus, she carried the family gift of transformation, able to change her form at will into flame, flowing water, a roaring lion, or a coiling serpent. This power would shape the entire course of her life.
The Fateful Prophecy
Thetis was so lovely that the two greatest gods of the sky and sea both desired her. Zeus pursued her as a bride, and his brother Poseidon courted her as well. Their rivalry ended abruptly when a prophecy came to light. The goddess Themis, keeper of divine law, warned that Thetis was fated to bear a son mightier than his father. Some tellings give this warning to the Titan Prometheus, who knew the secret and bargained with it. Zeus, who had himself overthrown his own father Cronus, understood the danger at once. To take Thetis to his bed would be to father the god who would one day cast him from his throne. Rather than risk the destruction of Olympus, the gods agreed that Thetis must never marry an immortal. They chose instead to give her to a mortal husband, so that her too-powerful son would live and die as a man.
Marriage to Peleus
The husband selected for her was Peleus, king of the Myrmidons and a hero of clean reputation. Thetis, a goddess wed against her will to a mortal, resisted him with every weapon her nature allowed. When Peleus seized her on the shore, she turned in his arms into fire, then into rushing water, then into a lion, and then into a hissing serpent, testing his courage with each shape. Peleus had been warned to hold fast no matter what she became, and he clung to her through every transformation until, exhausted, she returned to her true form and consented to be his wife. Their wedding took place on Mount Pelion, and the gods themselves came down to celebrate it. Yet one guest had been left off the list. Eris, the goddess of strife, arrived uninvited and rolled a golden apple among the guests, inscribed with the words "for the fairest." The quarrel it sparked among three goddesses led in time to the Judgement of Paris and to the Trojan War.
Mother of Achilles
From this marriage came Achilles, the greatest warrior of the Greek army at Troy. Knowing her son was doomed to a mortal span, Thetis tried to make him immortal. The famous late version tells that she dipped the infant into the river Styx, gripping him by one heel, so that every part touched by the water became invulnerable. The heel her fingers covered stayed unprotected, and that single weak point gave us the phrase "Achilles heel." An older tradition tells it differently: Thetis anointed the baby with ambrosia by day and laid him in the household fire by night to burn away his mortal parts, until Peleus caught her at it and cried out, breaking the ritual before it was finished. Whatever the method, her effort could not overturn the fate written for him.
Helper of Gods and Heroes
Thetis was known throughout mythology for her loyalty and her willingness to help others. When the smith-god Hephaestus was cast down from Olympus, it was Thetis and the Oceanid Eurynome who caught him and sheltered him for nine years in a cave beneath the sea, where he learned his craft. She once rescued Zeus himself: when Hera, Poseidon, and Athena plotted to bind the king of the gods in chains, Thetis summoned the hundred-handed giant Briareus, whose sheer strength scared the rebels into abandoning their scheme. She also gave refuge to Dionysus when the god fled into the sea to escape the attacking Lycurgus.
Her devotion shines brightest in the Iliad. When Achilles was dishonoured by Agamemnon, Thetis rose from the waves to comfort her weeping son and then climbed to Olympus to plead his cause with Zeus, winning his promise that the Greeks would suffer until they honoured her boy. Later, when Achilles lost his armour along with his friend Patroclus, Thetis went to Hephaestus and asked him to forge a splendid new set of weapons, including the great shield covered with images of the whole living world. She served her son to the end, mourning the death she had always known was coming, a goddess bound forever to the grief of loving a mortal child.