In Greek mythology, a Nereid loved by both Zeus and Poseidon. When it was revealed that Thetis was destined to bear a son who would be mightier than his father, the two gods gave her to Peleus, king of the Myrmidons. Unwilling to wed a mortal, Thetis resisted Peleus' advances by changing into various shapes, but Peleus finally captured and married her. Their child was Achilles. Some legends relate that she bore seven children, all of whom perished either when she tried to render them immortal or when she killed them as the offspring of a forced marriage.
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Silver-footed Thetis (ancient Greek Θέτις), disposer or "placer" (the one who places), is a sea nymph, one of the fifty Nereids, daughters of the ancient one of the seas in the historical vestiges of most Greek mythology. When described as a Nereid, Thetis was the daughter of Nereus and Doris (Hesiod, Theogony), and a granddaughter of Tethys. It is likely, however, that she was one of the earliest of deities worshiped in Archaic Greece, about whom written records do not exist except for one fragment, an early Alcman hymn which identifies Thetis as the creator of the universe and worship of Thetis as the goddess is documented to have persisted in some regions.
Pseudo-Apollodorus' Bibliotheke asserts that Thetis was courted by both Zeus and Poseidon, but she was married off to the mortal Peleus because of their fears about the prophecy by Themis (or Prometheus, or Calchas, according to others) that her son would become greater than his father. Thus, she is revealed as a figure of cosmic capacity, quite capable of unsettling the divine order (Slatkin 1986:12).
When Hephaestus was thrown from Olympus, whether cast out by Hera for his lameness or evicted by Zeus for taking Hera's side, the Nereids Eurynome and Thetis caught him and cared for him on the volcanic isle of Lemnos, while he labored for them as a smith, "working there in the hollow of the cave, and the stream of Okeanos around us went on forever with its foam and its murmur" (Iliad 18.369).
Thetis is not successful in her role protecting and nurturing a hero (the theme of kourotrophos), but her role in succouring deities is emphatically repeated by Homer, in three Iliad episodes: as well as her rescue of Zeus (1.396ff) and Hephaestus (18.369), Diomedes recalls that when Dionysus was expelled by Lycurgus with the Olympians' aid, he took refuge in the Erythraean Sea with Thetis in a bed of seaweed (6.123ff). These accounts associate Thetis with "a divine past—uninvolved with human events—with a level of divine invulnerability extraordinary by Olympian standards. Where within the framework of the Iliad the ultimate recourse is to Zeus for protection, here the poem seems to point to an alternative structure of cosmic relations and the reference relates to the religious concepts that greatly ante-dated the classical period.
An essential subordinate motif in the nature of Thetis, as a Nereid, one that links her with the dawn Titan Eos and with Aphrodite, is her liaison with a mortal lover: Thetis is the mother of Achilles by Peleus, king of the Myrmidons. Zeus had received a prophecy that Thetis's son would become greater than his father, the familiar mytheme of the Succession Prophecy. Therefore, in order to ensure a mortal father for her eventual offspring, Zeus and his brother Poseidon made arrangements for her marriage to a man, Peleus, son of Aeacus, but she refused him. Chiron, the wise centaur, who would later be tutor to her son by Peleus, Achilles, advised Peleus to find the sea nymph when she was asleep and bind her tightly to keep her from escaping by changing form. She did shift shapes, becoming flame, water, a raging lioness, and a serpent (compare the sea-god Proteus), but Peleus held fast. She then consented to marry him.
According to classical mythology, the wedding of Thetis and Peleus was celebrated on Mount Pelion outside the cave of Chiron and attended by the deities: there they celebrated the marriage with feasting. Apollo played the lyre and the Muses sang, Pindar claimed. At the wedding Chiron gave Peleus an ashen spear that had been polished by Athene and had a blade forged by Hepphaestus, and Poseidon gave him the immortal horses, Balius and Xanthus. Eris, the goddess of discord, had not been invited, however. In spite, she threw a golden apple into the midst of the goddesses that was to be awarded only "to the fairest." (In most interpretations, the award was made during the Judgement of Paris and eventually occasioned the Trojan War; by others such as Robert Graves, the imagery is considered misinterpreted and it is thought that it should reflect the selection of a king to be sacrificed in a sacred ritual).
In the classical myths Thetis worked her magic on the baby Achilles by night, burning away his mortality in the hall fire and anointing the child with ambrosia during the day, Apollonius tells. When Peleus caught her searing the baby, he let out a cry.
In a variant of the myth, Thetis tried to make Achilles invulnerable by dipping him in the waters of the Styx (the river of Hades). However, the heel by which she held him was not touched by the Styx's waters, and failed to be protected. In the story of Achilles in the Trojan War in the Iliad, Homer does not mention this weakness of Achilles' heel.
A similar myth of immortalizing a child in fire is connected to Demeter; compare the myth of Meleager. Some myths relate that because she had been interrupted by Peleus, Thetis had not made her son physically invulnerable. His heel, which she was about to burn away when her husband stopped her, had not been protected. Alternative interpretations assert that substitutes for the sacred king were sacrificed by fire (or water), putting off their ritual sacrifice for various numbers of years. Peleus gave the boy to Chiron to raise. Prophecy said that the son of Thetis would have either a long but dull life, or a glorious but brief life. When the Trojan War broke out, Thetis was anxious and concealed Achilles, disguised as a girl, at the court of Lycomedes. When Odysseus found that one of the girls at court was not a girl, but Achilles, he dressed as a merchant and set up a table of vanity items and jewellery and called to the group.
Only Achilles picked up the golden sword that lay to one side, and Odysseus quickly revealed him to be the warrior. Seeing that she could no longer prevent her son from realizing his destiny, Thetis then had Hephaestus make a shield and armor.
When Achilles was killed by Paris , Thetis came from the sea with the Nereids to mourn him, and she collected his ashes in a golden urn, raised a monument to his memory, and instituted commemorative festivals. According to alternative interpretations suggesting archaic traditions, Paris would have been the succeeding sacred king who was selected next by the three goddesses.
The one noted exception to the general observation resulting from the existing historical records, that Thetis was not venerated by cult, was in conservative Laconia, where Pausanias was informed that there had been priestesses of Thetis in archaic times, when a cult that was centered on a wooden cult image of Thetis (a xoanon), which preceded the building of the oldest temple; by the intervention of a highly-placed woman, her cult had been refounded with a temple; and in the second century AD she was still being worshipped with utmost reverence. According to Pausanias, "The Lacedaemonians were making war against the Messenians, who had revolted, and their king Anaxander, having invaded Messenia, took prisoners certain women, and among them Cleo, priestess of Thetis. This Cleo the wife of Anaxander asked for from her husband, and discovering that she had the wooden image of Thetis, she set up with her a temple for the goddess. This Leandris did because of a vision in a dream, but the wooden image of Thetis is guarded in secret."
In one fragmentary hymn by the seventh century Spartan poet Alcman, Thetis appears as a demiurge, beginning her creation with poros (πόρος) "path, track" and tekmor (τέκμωρ) "marker, end-post". Third was skotos (σκότος) "darkness", and then the sun and moon. A close connection has been argued between Thetis and Metis, another shape-shifting sea-power beloved by Zeus with a son greater than his father. This cosmogony is interesting not only because it takes up Near Eastern astronomical and theological speculation, but also because its first principles are the building-blocks of a race-track, reflecting the athletic preoccupations of Spartan society and education.
Herodotus noted that the Persians sacrificed to "Thetis" at Cape Sepias. By the process of interpretatio graeca, Herodotus identifies as the familiar Hellenic "Thetis" a sea-goddess who was being propitiated by the Persians.
In 2004, veteran actress Julie Christie portrayed Thetis in a short scene in the film Troy, in which her son Achilles, portrayed by Brad Pitt, featured heavily. During her entire scene, she was standing in an ocean pool.