Perseus and Medusa

Perseus's mother Danae is imprisoned in a tower by her own father after a prophecy warns that her son will kill him, but Zeus visits her as a shower of gold and Perseus is born regardless; both mother and child are later set adrift at sea in a chest and wash up safely on another island. Grown to manhood, Perseus is sent by a king who wants Danae for himself on what is meant to be a fatal errand: bring back the head of Medusa, one of three monstrous Gorgon sisters whose gaze turns any living thing that meets it instantly to stone, and the only one of the three who is mortal.

The gods equip Perseus for the impossible task: Athena gives him a polished bronze shield to use as a mirror so he need never look at Medusa directly, Hermes gives him winged sandals for flight, Hades lends a cap of invisibility, and he obtains a curved sickle-sword and a pouch to safely carry the severed head. Guided to the Gorgons' lair, Perseus watches Medusa's reflection in his shield, approaches her while she sleeps, and beheads her with a single stroke; from her blood spring the winged horse Pegasus and the warrior Chrysaor, both conceived earlier with Poseidon.

Before reaching the Gorgons at all, Perseus first had to locate them, information held only by the Graeae, three ancient sisters who shared a single eye and a single tooth between them, passing each back and forth. Perseus intercepted the eye as it changed hands and refused to return it until the sisters told him where to find both the Gorgons and the nymphs who could equip him with the tools he still needed.

Flying home with Medusa's head, Perseus passes over Ethiopia and finds the princess Andromeda chained to a rock as a sacrifice to a sea monster sent to punish her mother's boast of surpassing the sea nymphs in beauty. Perseus either turns the monster to stone with the Gorgon's head or kills it outright depending on the version, then marries Andromeda after resolving a dispute with a rival suitor by showing him the head as well, petrifying him and his allies at the wedding feast.

Returning at last to confront the king who sent him on the errand and who has been mistreating his mother in his absence, Perseus reveals the head one final time, turning the king and his court to stone, before giving the head to Athena, who mounts it on her aegis, the goddess's shield or breastplate, where it continues to serve as a protective, monster-deflecting emblem. An old prophecy that Perseus would kill his own grandfather is fulfilled almost accidentally afterward, when a discus he throws at athletic games veers off course and strikes an elderly spectator who turns out to be the grandfather who had once tried to have him killed.

Andromeda's mother Cassiopeia, whose boast had provoked the sea monster in the first place, along with her husband King Cepheus and the sea monster itself, were later placed among the stars as constellations, a detail ancient astronomers used to anchor the whole episode to a specific, permanently visible part of the night sky.