Cassiopeia was a woman in Greek mythology. She was the wife of Cepheus who was king of Ethiopia. They had a daughter called Andromeda. Cassiopeia was very beautiful but also very arrogant and vain. She thought that she was better than other people. One day, Cassiopeia said that her daughter Andromeda was more beautiful than all of the Nereids. The Nereids were sea-nymphs. When the sea-god Poseidon heard what Cassiopeia had said, he was very angry. He sent floods and a sea monster called Cetus to destroy Ethiopia. Cassiopeia and her husband Cepheus asked an oracle what to do. The oracle said that they must sacrifice their daughter so they chained Andromeda to a rock. A hero called Perseus came and rescued Andromeda. Later, they got married.