Definition: Thetis was the nymph mother of the Trojan War hero Achilles, and one of the 50 Nereid daughters of Nereus (son of Gaia [Earth] and Pontos [Sea]; whom Hercules seizes in order to extract information pertinent to his labors) and Doris (daughter of the Titans Oceanus and Tethys). She might not have been Achilles' mother, since the king of the Greek gods, Zeus, had been wooing her, but a prophecy that the son would be greater than the father made Zeus give her up. Instead, Thetis married a mortal king Peleus. They produced a son, Achilles, whom Thetis tried to make immortal. She dipped her infant son into the River Styx, holding him by the ankle, according to tradition. This made him invulnerable, except at the one weak spot where Thetis had held him. Peleus didn't agree with the treatment and Thetis left him. Thetis shows up in Homer's Iliad where she offers to get Achilles a new and better suit of armor and shield from the blacksmith of the gods, Hephaestus. Hephaestus was in her debt because Thetis and her sisters had cared for him when Hera threw him down from Olympus.