According to Greek mythology, Thetis was a sea nymph who gave birth to the famous hero Achilles*. The gods Zeus* and Neptune* both desired Thetis, but they did not pursue her because it was foretold that the son she bore would become greater than his father. So the gods arranged for Thetis to marry Peleus, a mortal.
The nymph did not wish to marry Peleus, however, and she changed into a sea monster and many other shapes to escape. Peleus held her until she changed back into a woman and agreed to marry him. Thetis bore him six sons, and she tried to make each son immortal by burning the child in a fire. She failed each time but tried again when she gave birth to a seventh son, Achilles. When Peleus discovered Thetis holding Achilles in the fire, he became so angry with her that she left him.
nymph minor goddess of nature, usually represented as young and beautiful
immortal able to live forever
In a later version of the myth, Thetis dipped Achilles into the river Styx to make him immortal, but she forgot to wet the heel by which she held him. Achilles was later killed during the Trojan Warf when the warrior Paris shot an arrow into his unprotected heel.