Cassiopeia boasted that 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, trying 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. Accordingly, Andromeda was chained to a rock at the sea's edge and left to be killed by the sea monster. Perseus arrives to kill Cetus, saves Andromeda and marries her.
Poseidon thought Cassiopeia should not escape punishment, so he placed her in the heavens chained to a throne in a position that referenced Andromeda's ordeal. The constellation resembles the chair that originally represented an instrument of torture.