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

Character Introduction
Tethys is the sea fairy of ancient Greek mythology, the daughter of Nejos, the friendly sea god, and Doris, the ocean god, and the unofficial leader of the fifty sisters of the sea fairy. When the Olympian god participated in a rebellion, he summoned the hundred armed giant Briareus to protect Zeus.
Zeus and Poseidon both pursued her, but Thetis prophesied that her child would be stronger than her father (Prometheus also knew this secret). After Zeus learned of this, he married her to Peleus, the son of Aeacos.
Another theory is that Thetis refused to rendezvous with Zeus because she was raised by Hera. Zeus became very angry and married her to a mortal. [1]
When Peleus pursued her, Thetis underwent various changes, first turning into fire, then into water, and finally into various wild beasts. Peleus held onto her tightly and did not let go until she returned to her original form. He married her on Mount Perion.

At their wedding, Eris, the goddess of discord, was not invited, so Eris decided to retaliate and threw the Golden Apple among the guests. The Golden Apple incident became the trigger for the Trojan War.
The son of Thetis and Peleus is the hero Achilles. Tethys wanted to keep her son alive, so she secretly burned him in the fire at night, intending to burn off his mortal body and give him eternal life. During the day, she wiped his body with divine milk, but Peleus caught her eye. Peleus saw his son struggling in the fire and cried out. Tethys was prevented from completing her goal, so she went back to the sisters. Although Achilles did not become an immortal, after training, he was completely invulnerable, with only his ankle being pinched and not refined, and could be injured. Eventually, he was shot here by Apollo and died.
Putting a child in a fire to prevent it from dying also happened to Demeter, the goddess of agriculture. She burned the son of Keraeus in the fire, but ultimately failed due to being seen.
Another theory is that Tethys held Achilles' ankle and soaked him in the Styx River, so his entire body could not be injured by weapons. Only the heel that did not soak in the Styx River was his weakness.
In the Iliad, Thetis hugged Zeus' knee with her left hand and stroked his chin with her right hand, praying to the divine king to restore glory to her son Achilles.