Cassiopeia, having boasted herself equal in beauty to the Nereids, drew down the vengeance of Poseidon, who sent an inundation on the land and a sea-monster, Kraken, which destroyed man and beast. The oracle of Ammon having announced that no relief would be found until the king exposed his daughter Andromeda to the monster, she was fastened to a rock on the shore.
Perseus, returning from having slain the Gorgon Medusa, found Andromeda, slew the monster, set her free, and married her in spite of Phineus, to whom she had before been promised. At the wedding a quarrel took place between the rivals, and Phineus was turned to stone by the sight of the Gorgon's head (Ovid, Metam v. 1).
Andromeda followed her husband to Tiryns in Argos, and became the ancestress of the family of the Perseidae through Perseus' and Andromeda's son, Perses.
After her death she was placed by Athena amongst the constellations in the northern sky, near Perseus and Cassiopeia. (Some say that she was chained to her throne and condemned to circle the North Star, sometimes hanging upside down in a very undignified position, as a warning.) Sophocles and Euripides (and in more modern times Corneille) made the story the subject of tragedies, and its incidents were represented in numerous ancient works of art.