I have to think this would be the most terrifying for a cephalopod, even nautiluses are contained within a shell, but aren't entirely encased. An exoskeletonned octopus would be like a knight cursed to never remove its armor.. kinda cool
After i posted i realised like lobsters and stuff coud feasibly meet an octopus, so they are probably familiar. Still terrifying though, if lobster was like human size
oo true horror, what if a nautilus's shell just kept growing, encasing the beast gradually and it becomes more and more snakelike and thin to labyrinth (verb) through its prison