Thetis/Definition

From Greek mythology, she is a sea nymph who was the mother of Achilles, sometimes described as silver footed, and one of the fifty Nereids. She has extended conversations with her son Achilles during the Iliad in which she tells him of two possible fates -- if he stays at Troy to fight, he will become famous for all time, but he will die; if he leaves, he'll live a long life but be obscure; Achilles chooses fame.

Source: Elizabeth Vandiver's Classical Mythology course.