Percy Jackson and the Olympians: Legends

The River Styx is a sacred river that separates the land of the living from the realm of the dead, one of five which run through the Underworld. Oaths made by this river brings something 'worse than death' to the oath bearer if not fulfilled. If anyone bathes in the Styx and survives, that person will bear the Curse of Achilles and become invulnerable to most physical attacks, excluding a small spot on their body that if struck will instantly kill them.