"Styx and Stones may break my bones but words will never hurt me." Well, they will if you swear by the River Styx and then break your oath.
The River Styx runs through the Underworld and is in the charge of the Goddess of that name. She’s the daughter of the Ocean and — with her consort PALLAS — has four formidable children: BIA (Force), CRATUS (Might), NIKE (Victory) and ZELUS (Zeal). They helped the Olympian Gods in their war against the TITANS, and you can probably tell just from their names that they were successful.
STYX was rewarded with her own ten-channel river, a palace in the Underworld and dominion over the Oaths of the Gods. Anyone swearing such an oath has to drink from her ice cold waters to bind their vow. Quite a chilling experience as these drinks can freeze the blood and destroy mere mortals.