He is a large kraken-like creature who was seen in the beginning with Scorpio, Leo, Lacerta and the Roc . Eris was demonstrating that she will make chaos to the Earth. She sends Cetus to attack the ships. While Cetus attacks Sinbad and his crew, Spike went after the sea monster and bites one of his tentacles. Cetus flings Spike into the air with a ripped tentacle in his mouth before being caught by a crewman. Cetus shoots his long spiked tongue and swallows one of the crew members. Sinbad sets a cannon and Cetus grabs it and it exploded inside him and he threw up the crew member with green slime everyplace. Cetus prepares to finish Sinbad by eating him, but he stabs his tongue with a sword and two sharp sticks on two masts were sliced off and they landed on Cetus, deliberately killing him. Cetus sinks into the ocean and grabs Sinbad.
Eris appears and puts Sinbad in a large air bubble. She explained her sea monster is deceased because of Sinbad. Sinbad apologized for injuring her sea monster due to the creature's attack. After they were finished talking about Sinbad himself getting the Book of Peace for Eris, she released him and Cetus appears again behind Eris and he shown fine, well, and alive. In another scene, Cetus was seen with the other monsters, while Eris was taking a bath and she wanted to see more action. She turns the ocean into ice and summons her Roc.
When Sinbad and his crew made into Tartarus, he and Marina both get inside. While falling, all of the remaining monsters, including Cetus, watch them from above as they continue falling.
- Although Eris said Cetus was dead, it shows Cetus was alive and well for a stange reason. Possibly since he is a constellation. He's capable of turning back into a constellation at will after his job was done.