Styx is the Demon Lady of Hate. She is the goddess of the River Styx in the Underworld, most often taking the form of a lovely nymph, but with cold black eyes, and sometimes tentacles in place of legs. During the Titanomachy, due to a hatred of the Titans for reasons forgotten, she sided with Zeus and sent her children to aid him. In return, Zeus blessed her and her river, and granted her a beautiful palace at the river's mouth, shielded from attack by other demons, who usually leave her in peace.
The Styx is the River of Immortality, and while its ice-cold waters have no effect on the immortal spirits and demons of the Underworld, it is said that they can grant mortals tremendous power, possibly invincibility... should they be strong enough to survive it. Contrary to commonly-held belief, Charon, the ferryman of the Underworld, does not carry the dead across the Styx, but rather the Acheron.
Hers are the souls of those whose hearts in life were frozen with hate, either for one person, or many, or all. Their souls are doomed to forever swim the waters of the frigid River Styx.
The Stygians are demonic water nymphs that inhabit the waters of the rivers in the Underworld. They mostly share the appearance of their mother.