Styx
Στύξ
The river by whose name even gods cannot break an oath.
It is the river dividing the world of the living from the dead — and also the name of the goddess who rules it.
A god who broke an oath sworn on the Styx lay breathless for a year and was banished from the councils of the gods for nine more.
Later tradition says this is the very river in which Thetis dipped the infant Achilles.
SourcesHesiod, Theogony