For 10 points, name this mythological river that forms the boundary between Earth and Hades.
The goddess of this name lives in a silver-columned grotto and is married to a son of Crius and Eurybia by whom she is the mother of Zelus, Cratos, Bia, and Nike. This wife of Pallas was the first immortal to assist Zeus during the Titanomachy. This goddess, the daughter of either Erebus and Nyx or Oceanus and Tethys, is the personification and namesake of a landmark that is visited by Iris whenever a god wishes to (*) swear the most solemn oath possible. It was either here or the Acheron where Charon performed his duties, and it was submersion in this body that made Achilles invincible. For 10 points, name this mythological river that forms the boundary between Earth and Hades.
The goddess of this name lives in a silver-columned grotto and is married to a son of Crius and Eurybia by whom she is the mother of Zelus, Cratos, Bia, and Nike.
the River {Styx}