In Greek mythology, Thetis is the mother of Achilles by Peleus, king of the Myrmidons. According to prophecy, Thetis's son would be greater than his father, so Zeus and Poseidon each resisted her beauty and made arrangements for her marriage to a mortal man.
According to legend, Thetis had tried to make Achilles invincible by dipping him in the river Styx, but forgot to wet the heel she held him by, leaving him vulnerable so he could be killed by a blow to that heel. (See Achilles' heel.) Homer, however, makes no mention of this; Achilles can not be a hero if he is not at risk. In an earlier and less popular version of the story, Thetis anointed the boy in ambrosia and put him on top of a fire to burn away the mortal parts of his body. She was interrupted by Peleus and abandoned both father and son in a rage. Peleus gave him to Chiron to raise.
Thetis was one of the gods behind the Oracle at Delphi, which she received from Gaia and gave to Phoebe.
Homer. Iliad; Apollonius Rhodius, Argonautica IV, 770-879.