The sea goddess, the daughter of the friendly sea god Nereus and the ocean goddess Doris, is the most virtuous of their daughters. The mother of Achilles, the hero.

Tethys is the sea fairy of ancient Greek mythology, the daughter of Nejus, the friendly sea god, and Doris, the ocean goddess, and the unofficial leader of the fifty sea fairy sisters. When the god Olympus was involved in the rebellion, he summoned the hundred armed giant Briareus to protect Zeus.
Both Zeus and Poseidon pursued her, but Tethys predicted that the child she gave birth to would be stronger than the father of the child (Prometheus also knew the secret). After knowing this, Zeus married her to Peleus, the son of Ajax.
Another said that Tethys refused to meet Zeus because she was raised by Hera. Zeus was very angry and married her to a mortal. [1]
When Peleus pursued her, Tethys changed from fire to water and finally to wild animals. Peleus held on to her until she changed back to her original appearance. He married her on Mount Pelion.
At their wedding, Eris, the goddess of discord, was not invited, so Eris decided to revenge and threw the golden apple among the guests. The golden apple incident became the fuse of the Trojan War.
The son of Thetis and Peleus was the hero Achilles. Tethys wanted to keep her son alive, so she hid from Peleus that she burned her son in the fire at night, intending to burn his body to make him immortal, and wiped his body with divine milk during the day, but Peleus spied it. When Peleus saw his son struggling in the fire, he could not help shouting. Tethys was prevented from fulfilling her purpose and went back to her sisters. Although Achilles could not become an immortal, he was invulnerable after training. Only his ankle could be hurt when he was pinched without training. Finally, he was shot here by Apollo and died.
It also happened to Demeter, the goddess of agriculture, to put the child in the fire to make it immortal. She burned the son of Cleos in the fire, and finally failed because of being seen.
Another said that Tethys held Achilles' ankle and put him in the Styx River to soak, so he could not be hurt by weapons. Only the heel that did not soak in the Styx River was a weakness.
In the Iliad, Tethys holds Zeus' knee with her left hand and touches his chin with her right hand, praying to the god king to let her son Achilles regain glory.