Once Andromeda's mother Kassiopeia said she was more beautiful than the Nereids. This angered the sea gods, and the god Poseidon sent floods and the sea monster Keto to them as punishment. An oracle told them that they had to sacrifice Andromeda to the sea monster, and because of that Andromeda was chained to a rock at the shore. But Perseus came to the place after he had killed Medusa. Perseus killed the sea monster, saved Andromeda and married her.
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