Oxidia

Oxidia was a Mechanicus forge world orbiting Karka's Sun.

Oxidia was tidally locked, so that half its surface always faced toward the blazing heat of Karka's Sun, its other half constantly shrouded in darkness. Mining operations could not be conducted in the direct light or direct darkness--the planet's Mechanicus overlords deemed the expenditure of resources necessary for such harvesting to be too much, especially considering the harvest they already got from the planet and the neighboring Karkason.

The Scarlet Sash
The Scarlet Sash was an equatorial, world-wrapping canyon, the sides of which provided inlets for noctilith strip mining. This canyon coincides with the "twilight band" of livable space on tidally-locked Oxidia. Therefore, the overwhelming majority of Oxidia's population and industry sheltered within the Sash. The materials mined from the Sash were significant enough to make Oxidia the primary noctilith export world for the entire Alenic Belt, and by consequence, the Lothairian Marches subsector as a whole.

Secret Origin
Main Article: The Steel-Walker War

At the end of the Steel-Walker War, it became clear that Oxidia’s true nature was that of a Necron redoubt world—a planetary storage depot for warrior-forms not dedicated to any one dynasty. Imperial command consulted with Fabricator General Kul Olonymich, and decided that the best course of action was the destruction of the entire planet.

In the firestorm that followed, Oxidia was scoured clean of all atmosphere and resources, it’s tomb complexes buried and annihilated even as they began to awaken. The world now orbits Karka’s Sun as a dead rock, though the fury of its death has jostled it free of its tidal lock, so that are now sunrises and sunsets on Oxidia, though none are left to appreciate them.