When Nereids overheard Cassiopeia she became very jealous. She complained to Poseidon, God of the Sea, and demanded that Cassiopeia be punished. Poseidon agreed and summoned a terrible sea-monster, Cetus. "Go to the coast of Cassiopeia's land," Poseidon directed, "and lay waste to the land, and kill the people, and kill the cattle." Cetus, in the form of a monstrous whale, set upon his mission of destruction and began the slaughter, working his way up and down the coast. The frightened people gathered and pleaded to their king to save them. Cepheus consulted an oracle--one with magical powers who could communicate with the gods when men sought their advice. The oracle told Cepheus that there was only one way to stop the slaughter: "You must offer your daughter Andromeda as a sacrifice." She was to be chained to the rocks on the coast and left for Cetus to devour. He made the bitter choice of sacrificing Andromeda, whereupon she was chained to the rocks and abandoned to await Cetus. When Cetus discovered the prize awaiting him, he left off his wholesale destruction of the land and began swimming toward the ledge where Andromeda was chained. But then a distant figure appeared in the sky. It was Perseus, the brave son of Zeus and Danae, just returning from a journey during which he had succeeded in killing the dreaded Medusa. It was when Perseus was flow over the coast of Æthiopia that he noticed Andromeda chained to the rocks by the sea, and not far away he could see Cetus rapidly nearing her. Down he swept to the girl's side. "Why are you thus bound?" he asked, overwhelmed by Andromeda's beauty. Andromeda told him the story of her boastful mother and the advice the oracle had given her father. Perseus quickly turned to Cepheus and said: "I can save your daughter from the sea-monster, but for my reward I demand Andromeda's hand in marriage, and a kingdom." Cepheus promised Perseus that he would have what he asked for, whereupon Perseus unsheathed his sword and leapt into the air to the attack. One thrust of his sword found a soft spot between the armored scales of the monster. Wounded, it twisted over on its side. Perseus then inflicted another deep cut, and another. Blood now colored the water red and soaked Perseus' winged shoes. Fearful of losing his ability to fly, he settled on a rock near the shore and waited for the sea-monster to attack again. As it did, Perseus' sword plunged deeply into the monster's evil heart.
Cepheus was King of Æthiopia (not present day Ethiopia), and the beautiful Cassiopeia was his Queen. Soon after their marriage, Cassiopeia bore her husband a daughter, Andromeda. Cassiopeia was vain and boastful. So great was her beauty and that of Andromeda, she said, that it surpassed even that of Nereids (the Sea-goddess).
In ancient Mesopotamia, these star were Tiamat, the cosmic dragon slain by the hero Marduk.