Styx

A strange phenomenon had left the moon of Styx actually orbiting Dis, and had therefore been used for defencive buildings and rocketry batteries capable of defending the moon.

A strange phenomenon during the planets creation had left the black moon of Styx in an orbit around the moon Dis and not the planet itself. Some ingenious soul of the Skull Bearers had decided to build interstellar artillery and defensive positions on Styx, able to defend Dis against even large attacks.