The River Styx was a river in Greek mythology which formed the boundary between Earth and the Underworld. It circles Hades nine times.
The gods respected the Styx and swore binding oaths by it. Zeus swore to give Semele whatever she wanted and was then obliged to follow through, resulting in her death. Helios similarly promised Phaëton whatever he desired, also resulting in his death. Gods that did not follow through on such an oath had to drink from the river, causing them to lose their voices for nine years, then being exiled from the council of gods for nine years after that. According to some versions, Styx had miraculous powers and could make someone immortal/invulnerable. Achilles may have been dipped in it in his childhood, acquiring invulnerability, with exception of his heel, which was held by his mother in order to submerge him. His exposed heel thus became known as Achilles' heel, a metaphor for a weak spot.