the.com/river styx

means The river in Greek mythology that separates the world of the living from the underworld, crossed only once — in one direction — by the dead.

from From Greek Styx, meaning 'hateful' or 'shuddering,' tied to the verb stygein, 'to hate.' Styx was also a goddess, daughter of the Titan Oceanus, and the river personified her name. The Greeks swore their most unbreakable oaths on her waters — even the gods, according to Hesiod, risked a kind of divine coma if they swore falsely by the Styx. The ferryman Charon and his one-way, pay-up-front toll come from the same myth cluster, which is how the river got its reputation as the underworld's final, non-negotiable checkpoint.