THE SCENE: When the gods of Olympus had to swear an oath, they would ensure compliance and truth by simultaneously drinking water from the river Styx. Because the consequences of foreswearing yourself after drinking Styx water were very grave indeed.
THE TEXT: But when strife and quarrel arise among the deathless gods, and when any of them who live in the house of Olympus lies, then Zeus sends Iris to bring in a golden jug the great oath of the gods from far away, the famous cold water which trickles down from a high and beetling rock. Far under the wide-pathed earth a branch of Oceanus flows through the dark night out of the holy stream, and a tenth part of his water is allotted to her. With nine silver-swirling streams he winds about the earth and the sea’s wide back, and then falls into the main; but the tenth flows out from a rock, a sore trouble to the gods. For whoever of the deathless gods that hold the peaks of snowy Olympus pours a libation of her water is forsworn, lies breathless until a full year is completed, and never comes near to taste ambrosia and nectar, but lies spiritless and voiceless on a strewn bed: and a heavy trance overshadows him. But when he has spent a long year in his sickness, another penance and an harder follows after the first. For nine years he is cut off from the eternal gods and never joins their councils of their feasts, nine full years. But in the tenth year he comes again to join the assemblies of the deathless gods who live in the house of Olympus. Such an oath, then, did the gods appoint the eternal and primaeval water of Styx to be: and it spouts through a rugged place.
– Theogany, Hesiod, 8th Century BC
