Thetis, the sea nymph, was devastated after the lost of her six sons, each who mysteriously died in infancy.
She longed to return to the sea, where she could roam free in the comfort of the waves. Thetis no longer wanted to be queen; she wanted to return to the simple life of a sea nymph.
However, as fate would have it, Thetis gave birth to her seventh son, Achilles. King Pelias and Thetis were rejoiced.
Determined not to lose her son, Thetis vowed to make Achilles invulnerable. Without telling her husband, she snuck out of her house with her son to the Underworld and dipped him into the River Styx. Thetis did not want to lose her baby to the swift waves, so she held him tight by one heel.
Meanwhile, at the house, King Pelias was distraught upon learning that Thetis and his son were gone. His wife had been acting strangly, and he was concerned for Achilles' safety.
He was relieved when they returned home, but kept a watchful eye on them. King Pelias pretended to be asleep.