Who was Cassiopeia?
Queen Cassiopeia, wife of King Cepheus of Ethiopia, was beautiful but also arrogant and vain. The boast of Cassiopeia was that both she and her daughter Andromeda were more beautiful than all the Nereids, the nymph-daughters of the sea god Nereus. This brought the wrath of Poseidon, ruling god of the sea, upon the kingdom of Ethiopia.
Accounts differ as to whether Poseidon decided to flood the whole country or direct the sea monster Cetus to destroy it. In either case, in the attempt to save their kingdom, Cepheus and Cassiopeia consulted a wise oracle, who told them that the only way to appease the sea gods was to sacrifice their daughter.