Gnossos

Gnossos is the homeworld of the Metallurgists' Idyll, the leaders of which live at the very core of the world.

Gnossos was once a lush, earth-like planet, though at the advent of the Metallurgists, all life was extincted. The Metallurgists mined the planet hollow, and it collapsed in on itself. Now, Gnossos is a literal shell, partially reconstructed to shield the all-important core from danger.