It is a lizard that appears to have adapted to the hazardous habitat of the Capital of the Unreturned, growing up in their eggshell and using it as a shield to keep them safe even from the "iron rain" of the 6th Layer. They're quite ferocious, but edible and can be eaten without too much work.
According to Lyza, they will continue to grow endlessly for as long as they live, and they appear to have a ridiculously long lifespan. The legendary delver claims to have encountered one with a towering frame of 30 meters. This elder Armorshell was strong enough that even the white whistle Lyza cherished for her phenomenal fighting ability failed to take it down, and she named it "Giant Armorshell" following her unsuccessful hunting.