When Cassiopeia boasted that her daughter Andromeda was more beautiful than the Nereids, this invoked the wrath of Poseidon who sent the sea monster Cetus to attack Æthiopia. Upon consulting a wise oracle, Cepheus and Cassiopeia were told to sacrifice Andromeda to Cetus. They had Andromeda chained to a rock near the ocean so that Cetus could devour her. Perseus found Andromeda chained to the rock and learned of her plight. When Cetus emerged from the ocean to devour Andromeda, Perseus managed to slay it. In one version, Perseus drove his sword into Cetus' back. In another version, Perseus used Medusa's head to turn Cetus to stone.