Thetis

Tethis is not to be confused with / is different from Tethys, the Titaness.

Jupiter and Thetis, by Jean Auguste Dominique Ingres

Thetis (Greek Θέτις) is a nereid (sea nymph) in Greek mythology. She is a daughter of Nereus and Doris. She is married to Peleus and with him the mother of Achilles. Some stories say that she dipped Archilles in the river Styx when he was a child to make him invulnerable. In Rick Riordan's series Percy Jackson and the Olympians, Percy is forewarned by Archilles ghost before he went into the Styx. Archilles mortal spot was his cursed heel.