The Bad Moon is the vast, leering celestial body worshipped as a god by the Gloomspite Gitz, the moon-crazed grots and their fungal kin. Part deity, part wandering catastrophe, it careens across the skies of the Mortal Realms on no course any astromancer can predict, a grinning face of rock and fungus whose arrival blots out the sun.
Wherever its sickly light falls, the world changes. Crops wither or sprout into forests of toxic mushrooms, black rain drives folk indoors, madness seeps into dreams, and the loonshrines of the grots overflow with fresh spite. To the Gloomspite Gitz this is glorious: they see the Bad Moon's glow as their god's favour, swarming from caves and ruins in vast hordes while it hangs overhead, then melting away when it wanders on. Prophets such as the self-proclaimed Loonking claim to read its will, though the Moon itself seems supremely indifferent to its worshippers.
For everyone else, the Bad Moon is a recurring natural disaster with a malicious sense of humour. Cities track its approach with dread, for its transits embolden every grot for leagues around, and no wall or ward has ever kept its influence entirely out.