Cetus is shown to be a gigantic reptilian creature covered in scales. It has an esca (the angler bit on an angler fish) and a horn on each side of his head and eyes resembling wavy slits that glow when agitated.
After eating part of the Island's Heart in Book 3, Chapter 4, the bonus scene of Act 7 reveals that Cetus has evolved into a hydra with two extra heads: one red and the other green. The red head has a pair of glowing green eyes, an esca, fins on both sides of its head, and spikes underneath its jaw. The green head has fins on both sides of its head, under its jaw, and on the back, spikes going down through its head, and a pair of yellow-green eyes.
It immediately has a hostile encounter with Jake, Sean, and possibly your character if he/she joins them on the speedboat. A premium option (if you choose to join Jake and Sean in the first place) gives you the opportunity to fight it with a Semtex found in the speedboat and a flare gun found in the marina (provided the latter isn't used before).
You are shown to be frustrated that Cetus seemingly can't die and always shows up unexpected. In Book 3, Chapter 4 you yell "Damn you, Cetus! Why can't you just leave us alone?"
At the end of Book 3, Chapter 7, you vow to hunt down Cetus and kill it once and for all, and in the end, you succeed.
- Michelle's Catalyst Idol shows her getting killed by Cetus. It was later revealed that this happened in one of the past timelines. In a different timeline, both she and Raj were killed during Cetus's attack on the Vaanti village.
- Jake's nickname for Cetus is Freakachu, a reference to the Pokémon character Pikachu, because of its electric attacks.
- If you didn't fight him in Book 1, it will have its scarred appearance in Book 3.
- If you unlock the clue of the Endless's musings in Book 3, Chapter 10, it is revealed that Cetus was brought to La Huerta, along with the other guardians, to maintain order. However, at some point, the creature lost it's sanity and became hostile and aggressive.