Styx was a Titan goddess in Greek mythology, daughter of the Titans Oceanus and Tethys, thus being one of the Oceanid sisters. She was the goddess of the River Styx, and wife to the Titan Pallas, with whom she had four children; Zelus, Nike, Kratos, and Bia. Styx fought on the side of Zeus and the Olympians during the Titanomachy, the great war between the Titans and the Olympians, the latter emerging victorious. To honour her, Zeus gave her name to the binding oath that was taken by the gods.
The River Styx was the boundary between the realm of Earth and the Underworld. In its waters, the great hero Achilles was submerged when he was an infant by his mother Thetis, thus making him invincible; however, as she held him by his heel, that was the only spot on his body that was vulnerable. This is how the phrase Achilles' heel was created to signify a vulnerable spot.