For a pack of werewolves to hunt, it must have prey. Often, the half-seen faces in the shadows are the Uratha themselves, stalking human prey. Other times, those half-hidden forms belong to less-salubrious creatures.
Werewolves form packs to hunt things far more powerful than they are — immense spirits of entire buildings or even cities, shartha that would be the Plague King, or human institutions that have no throat to cut and no body to kill.
Out in the darkness, once kept prisoner by the Moon, lurk things that even the Uratha do not know how to hunt. These Moon-Banished creatures, the idigam, are spirits of concepts that do not exist, forced into a life of perpetual change just to exist.
Each idigam is unique, shaped by its nature and by the anchor that gives it a point of stability in the world. Each idigam hates Father Wolf for imprisoning it on a lifeless rock. Each idigam is quite content to take that frustration out on his bastard children.