Answer:
Modesty.
Step-by-step explanation:
She was the Queen of Aethiopia and the wife of King Cepheus. Her arrogance and vanity led to her downfall.
She said to everyone that her daughter Andromeda and herself were more beautiful than the nymph-daughters of the god of the sea Nereus. Such claims made Poseidon furious and he decided to flood the whole country.
Both king and queen consulted an oracle to see what could they do to avoid that, the oracle told them that in order to stop the wrath of Poseidon they must sacrifice their daughter. They did that by tying her to a rock and leaving her there to be killed by the sea monster Cetus. But the Greek hero Perseus arrived and save Andromeda, then they marry.
Nevertheless, Poseidon punishes Cassiopeia by placing her in heaven tied to a throne in a similar position as she had tied Andromeda.
I hope this answer helps you.