Emperorshell

The Emperorshell (ダイオウカブリ) is a creature living on the 6th Layer of the Abyss, first officially documented by the White Whistle Delver Lyza, "The Annihilator", having received its name from her.

Description
Emperorshells are large lizards that appears to have adapted to the hazardous habitat of the 6th Layer, growing up in their eggshell and using it as a shield to keep them safe even from the "iron rain" found there. They are quite ferocious, but edible, and can be eaten without too much preparation.

According to Lyza, they will continue to grow endlessly for as long as they live and they appear to have a ridiculously long lifespan. In her notes, she claims to have encountered one with a towering frame of 30 meters. This Emperorshell was strong enough that even Lyza, noted for her phenomenal fighting ability, seemingly failed to take it down, leading to her naming it "Emperorshell" following her unsuccessful hunting.

Trivia

 * According to Lyza, the juvenile form of the Emperorshell is very tasty and whether or not the same goes for the one of enormous size she encountered, too, is a question she would leave for those who come after her to answer.
 * The habitat of the Emperorshells rests beyond a depth of 14,000 meters, lying in the 6th Layer, accompanied by flaming iron rain.
 * The book that Nanachi used to read to Mitty contains a folk song apparently based on the White Whistles. In the book, there is a passage that describes the habitat of the Emperorshells that the group encounters: "Unnoticed, the shooting stone leaves a trail which falls as red-hot iron rain. The ice covered trees transform into clouds. In the gap in the sky, a big turtle with its neck stretched out is grouped with others, looking up at the golden sky."