Styx

if I had had the sense to foresee all this, when Eurystheus sent him to the house of Hades, to fetch the hell-hound from Erebos, he would never have come back alive out of the deep waters of the river Styx.

Sleep was pleased when he heard this, and answered, "Then swear it to me by the dread waters of the river Styx;

Hera trembled as he spoke, and said, "May heaven above and earth below be my witnesses, with the waters of the river Styx - and this is the most solemn oath that a blessed god can take - nay, I swear also by your own almighty head and by our bridal bed - things over which I could never possibly perjure myself - that Poseidon is not punishing Hektor and the Trojans and helping the Achaeans through any doing of mine;

You will find it near the place where the rivers Pyriphlegethon and Cocytus (which is a branch of the river Styx) flow into Acheron, and you will see a rock near it, just where the two roaring rivers run into one another.

May heaven above and earth below be my witnesses, with the waters of the river Styx - and this is the most solemn oath which a blessed god can take - that I mean you no sort of harm, and am only advising you to do exactly what I should do myself in your place.

Then we must further taboo in these matters the entire vocabulary of terror and fear, Cocytus named of lamentation loud, abhorred Styx, the flood of deadly hate, the people of the infernal pit and of the charnel-house, and all other terms of this type, whose very names send a shudder through all the hearers every year.