Greek Mythology: the Nymph, Thetis
Thetis was the most prominent of the fifty Nereids, the daughters of Nereus and Doris (early deities of the sea who were never associated with Mouth Olympus). As the daughter of gods, it would seem only fitting that she, too, would be worshiped as a goddess - and, it seems, that there were points in history where she was revered as a minor goddess of the sea, along with her divine parents.Yet, as the Greek pantheon of gods came to be centered solely around Mount Olympus, it seems that it became much more common for the role of Thetis and her sisters to be reduced to that of sea nymphs (a nymph being a spirit of nature common in Greek myths and legends). The Nereids quickly came to be seen as a part of the retinue of Poseidon.
Thetis was never a part of the court of Mount Olympus, yet there were many occasions where her actions earned her the gratitude of the gods. Thetis had, at different points, offered protection and safe haven to both Dionysus (God of Wine and Merry Making) and Hephaestus (God of Fire and the Forge) when they found themselves in need. She had even come to the aid of Zeus, himself, when the other gods of Mount Olympus threatened to depose him, and bind him in chains.
The Golden Apple of Discord
Thetis was as beautiful as any other of her kind - so, it seems only natural that she would become an object of desire. Both Zeus and Poseidon had intentions toward the nymph that were not exactly pure - yet, ultimately, neither was willing act on their desire.
The reason for this was simple. It just so happened that there was also a prophecy concerning Thetis - one that stated that any son born to her would grow to be more powerful than his own father. Thanks to this prophecy, Zeus realized quickly that any child he bore with Thetis would grow to pose a significant threat to him - maybe even resulting in him suffering much the same fate as his own father, the Titan Cronus. So, in an uncharacteristic display of restraint for the infamously promiscuous deity, Zeus chose to restrain himself.
This was not enough of a precaution for the ruler of Mount Olympus, However. He, instead, also made the decision to arrange for the nymph to be married off to a mortal man - on the assumption that a son who grew to be greater than a mere mortal still would not pose any real threat to a god. The future husband chosen for her was Peleus, a Greek hero who was once the companion of Heracles (better known as Hercules). Peleus approached Thetis with his intention to marry her, but she refused him. It was another deity of the sea, Proteus, who revealed to Peleus how he could restrain the sea nymph, and overcome her ability to change her form. Approaching Thetis once more, Peleus held onto the sea nymph as she struggled against him. Thetis assumed a variety of forms as she tried to escape, but Peleus was able to maintain his grip on her. Finally, Thetis gave up her struggles - once more assuming her natural form, she relented and accepted the marriage arranged for her.
Thetis was, naturally, unhappy with the idea of being forced to marry, though - and, even more so at the idea of being forced to marry a mortal man. But, Zeus tried to console her by promising to make her wedding ceremony one that would be remembered. The wedding of Thetis to the mortal Peleus was, therefore, a lavish affair attended by all of the gods of Mount Olympus. All but one, at least. Eris, the Goddess of Discord, was turned away, since it was feared that she would only try to ruin the ceremony if she were allowed to attend. In retaliation, played on the well-known vanity of the gods of Mount Olympus by fetching a golden apple from Hera's own orchard, and inscribing the single word, 'Kallistai' ('to the fairest'). Eris, then, tossed this apple into the crowd of deities, knowing that there would be more than one eager to claim it as their own. She was right, of course - and, the results of this little trick lead directly to the story of the Judgement of Paris.
Despite the best efforts of the Goddess of Discord, though, the wedding still went ahead without any further problems - and, Peleus and Thetis were married.
The Mother of Achilles
Thetis tolerated her mortal husband, yet seemed to fear the inherent weakness of mortality - as seen through the eyes of an immortal being. She had no desire to see her own children crippled by this weakness - to see them age and die, or to see them cut down before their time. So, as some versions go, as each of her children was born Thetis quickly took it up and cast it into a fire - hoping to burn the mortality out of them, and to leave behind the immortality offered by her own blood. Her plans failed, though, and none of her children survived the process.
It was as Thetis was about to subject her sixth child to this treatment that her husband, Peleus, was finally able to catch her in the act. He snatched the newborn child from its mother, and was ultimately able to convince her to raise it was her own.
In other versions, though, it seems that Thetis' desire for a child that shared her own immortality is treated a little more reasonably (and, a little more sympathetically, perhaps), by simply removing the existence of any previous children from the tale. In these versions, Thetis only had one son - and, it was as she was preparing to subject him to the process intended to burn away his mortality that the horrified Peleus came upon them, and snatched the child from his mother.
At the same time, though, her young son, who was named Achilles, was also often portrayed as being invulnerable to any wounds in the stories based around him. In some versions, Thetis' plans to burn away her child's mortality showed every indication of working, as she anointed his body with ambrosia (the food of the gods) and place him on top of a fire - only to be interrupted by an outraged Peleus. In others, Achilles' invulnerability was achieved later, when Thetis took her young son to the river Styx (the boundary between the mortal world and the Underworld of Hades) and submerged him in its water - holding him by the heel of one foot to keep him from being carried away by its current. In all versions of the story, though, it seemed to be a common trait for the heel of one foot to be left mortal and, therefore, vulnerable. To further confuse matters, though, in Homer's Iliad, Achilles is presented as entirely mortal, and very much capable of being wounded.
Achilles grew to be a healthy and strong child - and, in time, Thetis grew to love him as a mother should. Yet, Thetis always feared for her son. He was, after all, the subject of a prophecy of his own - one that stated that he would either live a long, but dull, life or one that was both glorious and short. When news of the outbreak of the Trojan War reached Thetis, she began to fear that fate might be leading her son toward the later of the two - and, so, she made an attempt to conceal him from anyone who would try to recruit him and send him off to battle. She had Achilles disguise himself as a girl in the court of Lycomedes, the king of Scyros - but, he was eventually found out by Odysseus. Knowing that she could not prevent what fate had in store for her son, she went to Hephaestus and had the God of the Forge craft her son a shield and a suit of armor finer than anything that could be crafted by a mortal.
Yet, despite her best efforts, Thetis' beloved son was one of many from both sides killed during the Trojan War. As Thetis mourned for Achilles, she was joined by all of her sisters. Her final role in the story of Troy was to gather her her son's ashes in a golden urn, and to raise a monument in his memory.