Cepheus was king of Aethiopia when he heard his wife Cassiopeia boast that her daughter was more beautiful than the Nereids. Shocked, he tried to silence his wife, but it was too late. The father of the Nereids, the sea God Nereus, heard Cassiopeia's prideful boast and brought his grievance to Poseidon. Poseidon ruled in favor of Nereus and sent Cetus, a huge sea monster, to ravage the coasts of Aethiopia. It destroyed villages, kept fishermen off of the sea and caused huge floods that killed hundreds at a time. A cry went up from the people for Cepheus to remedy the situation and to appease the Gods. Cepheus, desperate, traveled to the oracle of Apollon (or Zeus) to hear how he could solve the suffering of his people. The Oracle told him that Nereus would only be appeased when he sacrificed his daughter to Cetus.
Stricken with grief, Cepheus raised his hands to the sky--the position he was immortalized in--and prayed for another resolve of the situation that would not lead to the death of his much beloved daughter. The Theoi, however, remained silent. Cepheus resisted the oracle's message as long as he could, but eventually, the anguish of his people became too much for a king to withstand. In other versions of the myth, Androméda (like Iphigeneia) offered herself up to be sacrificed, as she realized her life was not worth the lives of all those who were dying now.
Androméda was chained to the cliffs near the palace, and awaited her faith there, while both Cepheus and Cassiopeia looked on with immense sadness. Thankfully, Androméda was rescued from her fate by Perseus, on his way back from defeating Médousa. King Cepheus hosted a huge wedding banquet at his palace to celebrate the wedding. There was one problem, however: Androméda had already been promised to Phineus, Cepheus' brother. While the celebrations were in progress, Phineus and his followers bursted in, demanding that Androméda be handed over, which Cepheus refused to do--too grateful to Perseus for rescuing his daughter from certain death. Ovid has described the battle that ensued in the Metamorphoses, but before the battle begins, we first get a speech by Cepheus to his brother, who begs him to let his claim to Androméda go:
"Hold, brother, hold; what brutal rage has made your frantick mind so black a crime conceive?
Are these the thanks that you to Perseus give? This the reward that to his worth you pay, whose timely valour sav'd Andromeda? Nor was it he, if you would reason right, that forc'd her from you, but the jealous spight of envious Nereids, and Jove's high decree; And that devouring monster of the sea, that ready with his jaws wide gaping stood to eat my child, the fairest of my blood.
You lost her then, when she seem'd past relief, and wish'd perhaps her death, to ease your grief
With my afflictions: not content to view Andromeda in chains, unhelp'd by you, her spouse, and uncle;
will you grieve that he expos'd his life the dying maid to free? And shall you claim his merit?
Had you thought her charms so great, you shou'd have bravely sought that blessing on the rocks, where fix'd she lay: but now let Perseus bear his prize away, by service gain'd, by promis'd faith possess'd;
To him I owe it, that my age is bless'd still with a child."
Phineus refused to listen to reason, and threw a spear at Perseus, who barely managed to dodge it. After that, all hell broke loose. Perseus cut down many of his attackers, turning the remainder to stone by showing them the head of Médousa. Eventually, he hailed victorious, and got to carry off his bride. In doing so, he left Cassiopeia and her husband to the fate of Poseidon, who would still have His revenge. As such, He took both Cassiopeia and Cepheus up into the sky and placed them near each other in the heavens. Poseidon placed Cassiopeia close to the North Celestial Pole on her throne, spending half of her time clinging to it so she does not fall off. In old portraits of the constellation, she is seen as either tied to her throne--which most often resembles a torture device--or desperately clinging to it. Later on, she was depicted as holding a mirror (or palm leaf) to show her vanity.
However, because Cepheus had nothing to do with Cassiopeia's original declaration, because he had done everything in his power to make things right afterwards, and he had plead his case to the Gods again and again, Cepheus was placed into the sky unchained--either regally on his throne or with his hands raised in pious prayer--and a little further away from the pole. He still circles it in punishment, but his position is a lot less precarious than that of his wife.
The constellation Cepheus is visible at latitudes between +90° and −10°, and best visible at 21:00 (9 p.m.) during the month of November, the same as the constellations of his wife and daughter.