The Ketos Aithiopios or Ethiopian Sea Monster was sent to plague modern day Ethiopia. The kingdom's queen, Kassiopia was so vain she compared her beauty to that of Aphrodite.  Aphrodite, in turn, complained to Zeus who in turn convinced Poseidon to send the monster to terrorize the kingdom.  The land's king, Kepheus went to Delphi and was told that the way to get rid of the monster was to sacrifice Kepheaus & Kassiopia's daughter Andromeda.  The king and queen chained her to a rock and waited for the monster to come.  While waiting,  The hero Perseus arrived and learned of the horrible fate that would be befallen Andromeda without his intervention.  Perseus had actually come back from killing Medousa and taking her head.  When the monster Ketos Aithiopias arrived to claim its victim, Perseus removed from his pouch the head of Medusa using it to turn the monster to stone.  Perseus then freed Andromeda and married her.