Life on Earth
Cetus was created by the sea god Poseidon to punish the region of Ethiopia for the the insolence of its queen Cassiopia, who boasted that her daughter Andromeda surpassed the Nereid nymphs in beauty. As the sea monster destroyed the shores of the region and terrorized its people, Cassiopia and her husband Cepheus were advised by their court oracle to sacrifice their daughter to him in order to appease Poseidon. The royal couple agreed by chaining Andromeda to a rock next to the sea so Cetus could prepare to attack and devour her. Fortunately for the princess, the hero Perseus stumbled upon the trouble while flying back to the island of Serifos with the head of Medusa. Instantly falling in love with her, Perseus battled and killed Cetus by either stabbing him a few times or using the gorgan's head to turn him into stone while freeing Andromeda from her bonds.
Both the princess and her hero married on that very day, and Cetus died from his wounds into as he sank into the sea with his blood spreading all over the water. For this reason, the sea where his battle with Perseus took place became known as the Red Sea, which was later parted for Moses' mission to deliver the Hebrews out of slavery in Egypt. Upon his death, Cetus was placed as a constellation as a reminder of Perseus' bravery and a warning for people about anger and jealousy. But the monster continued to be a threat to humankind when placed as one of the star-outlined servants to the chaos goddess Eris in her nighttime-mirroring realm of Tartarus, which enabled him to be sent back down Earth whenever she needed him to help do her bidding in unraveling the planet into "glorious chaos".
Cetus was first seen in the beginning of the film with Scorpio, Leo, Lacerta and the Roc as Eris was demonstrating how she will make chaos to the Earth, planning it by stealing the legendary Book of Peace as she spotted it being transported to Syracuse on a noble ship captained by the city's Crown Prince Proteus, with a pirate ship captained his childhood friend Sinbad on his tail. Delighted by what fun this brought her, she sent Cetus down to the middle of the fray to attack the ships. While coming aboard Proteus' ship trying to reach for the Book, the sea monster was bitten in the tentacle by Sinbad's dog Spike, so he flung the dog into the air with a ripped tentacle in his mouth before being caught by a crewman. Cetus shot his long spiked tongue and swallowed one of the prince's crew members, but Sinbad himself set a cannon for the monster to swallow as well, causing it to explode inside him and him to throw up the crew member with green slime everywhere. Angered, he prepared to finish Sinbad and Proteus by eating him, only to get his tongue stabbed onto the deck by a sword while the duo sliced off two sharp sticks from the masts and had them land on Cetus, deliberately killing him. Cetus sunk into the ocean, but not before grabbing Sinbad and taking him along to the depths.
Eris appeared and put the sinking sailor in a large air bubble, explaining that Cetus was deceased because of him. Sinbad apologized due to the creature's attack, but after they were finished talking about Sinbad himself getting the Book of Peace for Eris, she released him and Cetus appeared again behind her, alive and well. In another scene, he was seen with the other monsters, while Eris was taking a bath and she wanted to see more action. She turned the ocean into ice and summoned the Roc.
When Sinbad and his crew made into Tartarus, he and Marina both got inside. While falling, all of the remaining monsters, including Cetus, watched them from above as they landed to the bottom.
- Although Eris said Cetus was dead, it shows Cetus was alive and well for a stange reason. Possibly since he is a constellation. Alternately, Tartarus is the Greek analogue to Hell, so it’s possible that was his soul. Cetus is also capable of turning back into a constellation at will after his job was done.