Phobos

Phobos is the inner moon of Mars, a gun-studded orbital fortress that covers the Red Planet. Vaults of forbidden Mechanicum relics sit under its batteries and open only on the Fabricator-General's word. Loyalist Skitarii fought for those vaults in the Schism of Mars, and Kubik later used Ork teleport gear to fling the moon across Martian orbit.