Adventures in Theia
“The Unseen Moon” and “The Moon of Ill Omen,” Phobos is an erratic dark body in the heavens, whose comings and goings defy prediction by astronomers and astrologers.
Phobos cannot be seen by the naked eye, but its movements are discernible by the stars it blots out as it move across the night sky. It usually appears on average 5 days per month, but has been known to cross the night sky every day of the month, and has disappeared for years at a time. When Phobos is absent from the night sky it said that Phobos descends and roams Theia seeking to spread chaos and destruction in his wake.
Phobos is reported to be worshiped by the fell things of the earth — vampires, the undead, and the spawn of Shadow. Nasty rumors about cannibalistic tribes of sub-humans at the fringe of civilization are also said to propitiate the dark god with blood sacrifices.