Styxn.

Styxlate 14c., the Greek river of the Underworld, literally "the Hateful," cognate with Greek stygos "hatred," stygnos "gloomy," from stygein "to hate, abominate," from PIE *stug-, extended form of root *steu- (1) "to push, stick, knock, beat." Oaths sworn by it were supremely binding and even the gods feared to break them. The adjective is Stygian.