According to Greek mythology, Styx was a a goddess, daughter of Oceanus and Thetis. She had been especially honored by Zeus for the part she had played in The Battle of the Giants. However, Styx lived away from the other gods, in a palace that was built on huge rocks out of which water was flowing. According to Homer, the waters of the palace led to the gates of Hades.
Styx was also one of the great rivers of the underworld.
The ancient Greeks believed that, in order to travel to the underworld, they had to cross the River Styx. The souls were transported across by Charon, in a rickety boat with the price of two bronze coins that were placed by the relatives of the deceased, under the tongue or over closed eyes.
Most importantly, the waters of Styx were used for official vows.
When a god wanted to swear an oath, Zeus sent Iris, the only immortal to enter Styx’s home, to get a jug of water from the Styx
and go to Mount Olympus to become a "witness" of the oath.
If this god trampled upon his oath, he would receive a terrible punishment: he would live a whole year speechless and languished, without ambrosia and nectar.
The waters of Styx were so hot that anyone who drank from the water of the Styx, would die and any metal dived into the waters of the Styx would melt. Only the hooves of the horses could survive the hot temperatures, for this reason the gods drank in cups made out of horse hoof.
In the waters of Styx, the Nymph Thetis dipped her son Achilles in order to render him invulnerable and immortal.
Only his heel, the famous the "Achilles heel", was not soaked with water because it was the spot where his mother held him.
Later, in the battle of Troy, Paris would discover this vulnerability and inflict a fatal wound with a poisoned arrow in that heel.