Cepheus was born to Belus, the king whose name echoed the Canaanite storm-god Baal, and to Achiroe, the daughter of Nilus, the river god whose waters made Egypt green. His brothers were Danaus, who would flee to Argos with his fifty daughters; Aegyptus, whose fifty sons would be murdered on their wedding night; and Phineus, who would one day try to claim Cepheus' own daughter as his bride. Some traditions called Cepheus the son of Phoenix or of Agenor instead, but the oldest and most widely accepted genealogy made him a son of Belus, a prince of the house that linked Egypt to Greece through the wandering Io. He was called Iasid Cepheus in some sources, a title that traced his blood through King Iasus of Argos, the father of Io, back to the primordial kings of the Peloponnese.

Cepheus left the lands of his father and travelled to Aethiopia, the kingdom at the southern edge of the known world, a land of burning sun and dark-skinned peoples that the Greeks linked with both wealth and mystery. There he became king, and there he married Cassiopeia, a woman of extraordinary beauty whose origins the poets did not preserve. She bore him at least one child, a daughter named Andromeda, whose face was so lovely that poets would sing of it for centuries. The family ruled from a palace whose wealth was said to be immense, drawn from the gold and incense of the southern lands. Some later writers placed their kingdom not in Aethiopia but at Joppa on the coast of Phoenicia, where a rock was shown to visitors with the marks of Andromeda's chains still visible upon it. Either way, the palace was grand, the kingdom was prosperous, and the disaster that came upon it began not with war but with a single sentence spoken by the queen.

Cassiopeia looked at her daughter, or perhaps at her own reflection, and declared that the beauty of the house of Cepheus surpassed that of the Nereids, the fifty sea-nymph daughters of Nereus who were the companions of Poseidon, the god of the sea. The Nereids went to their master and demanded vengeance. Poseidon, never slow to anger when his honour was slighted, struck the coast of Aethiopia with a flood and sent a sea monster named Cetus to ravage the land. The creature rose from the depths like a moving mountain, its jaws large enough to swallow ships whole, and it laid waste to harbours and fields. The people of Cepheus' kingdom cried out for salvation, and the king, desperate, did what every Greek ruler did in crisis: he consulted an oracle.

The oracle of Ammon lay in the Libyan desert, a sanctuary of the Egyptian god whom the Greeks identified with Zeus. Cepheus travelled there, or sent emissaries, and the priestess gave a chilling answer: the land would be spared only if Cepheus sacrificed his daughter Andromeda to the monster. The king returned to his city torn between love for his child and duty to his people. His subjects, terrified by the flood and the beast, demanded that he obey. Cepheus yielded. He had Andromeda stripped of her royal garments, chained naked to a rock on the shore, and left for Cetus. The image of the princess bound to stone, her chains cutting into her wrists while the spray rose around her, became one of the most enduring scenes in Western art, painted by Titian and Rubens and countless others across two thousand years.

Salvation came from the sky. Perseus, the son of Zeus and the princess Danae, was returning from his quest to slay the Gorgon Medusa, the creature whose face turned all who looked upon it to stone. He flew over the coast on winged sandals given to him by Hermes, the messenger god, or rode the winged horse Pegasus that had sprung from the blood of the slain Gorgon. Looking down, he saw Andromeda chained to the rock and the monster rising from the sea. He fell in love at first sight. He descended and struck a bargain with Cepheus: he would save the princess in exchange for her hand in marriage. Cepheus, grateful beyond measure, agreed. Perseus then turned to face Cetus. Some say he killed the beast with his sword. Others say he simply uncovered the Gorgon's head and the monster turned to stone in an instant. Either way, Andromeda was free. Perseus cut her chains, wrapped her in his cloak, and claimed her as his bride.

The wedding was not peaceful. Andromeda had been betrothed before her sacrifice to her uncle Phineus, the brother of Cepheus, and Phineus arrived at the feast with an armed band to reclaim her. Cepheus stood aside, or tried to mediate, but the dispute turned violent. Perseus, outnumbered, did what he had done to the Gorgon: he raised Medusa's severed head. Phineus and his allies froze where they stood, turned to stone in the middle of the palace, their faces forever fixed in expressions of rage and disbelief. In some versions, Cepheus and Cassiopeia were also turned to stone because they did not look away in time, though the more common tradition says they survived to see their daughter depart.

After the wedding, Perseus and Andromeda remained at the court of Cepheus for a year or more. During that time, Andromeda gave birth to their first son, whom they named Perses. Cepheus had no male heir of his own, and Perseus, understanding the old king's need, allowed him to adopt the infant. Perses grew up in Aethiopia and gave his name to the Persians, or so the Greeks believed. Every Persian king from Cyrus to Darius to Xerxes was thus, in the Greek mythic imagination, a descendant of Cepheus, King of Aethiopia, through the line of Perseus and Andromeda. It was a claim that made the wars between Greece and Persia into something more than a clash of armies: it made them a family feud, a quarrel between cousins who shared the same divine ancestor.

When Cepheus died, Athena placed him among the stars as the constellation that bears his name. He was set near the north celestial pole, close enough to his wife Cassiopeia and his daughter Andromeda that the family is still visible together on winter nights, though his crown is formed of faint stars that only the patient eye can trace. Eratosthenes, the Greek astronomer who wrote the Catasterismi in the third century BCE, described him as a man wearing a crown, upside down with respect to the ecliptic, his head near the pole and his feet stretching toward the Milky Way. Ptolemy included him among the forty-eight constellations of the Almagest, and he remains one of the eighty-eight recognised by modern astronomy. The constellation contains some of the most remarkable objects in the sky: the Garnet Star, one of the largest known stars; Delta Cephei, the prototype of the Cepheid variable stars that Edwin Hubble used to measure the distance to other galaxies; and the Wizard Nebula, a cloud of gas and dust where new stars are being born. Every time an astronomer measures the distance to a distant galaxy using a Cepheid variable, they are using a star that bears the name of the king who once chained his daughter to a rock.