what about the goddess of river styx!!!!
I've mentioned her on the page about goddess Nike, but I forgot to add her to the list of Greek goddesses, my bad!
She was the goddess of the underworld river Styx. She was the daughter of Oceanus and Thetys. Being the first one to have come to Zeus' aid (together with her children), in his battle against the Titans, he bestowed upon her a great honour: all the solemn oaths were made in her name. Whenever a god made an oath on her name, goddess Iris would fetch a cup of water from the river Styx, and the god taking the oath would pour it out.