From Dmitri: 

There are a lot of legends among werewolves about their origins. The most popular legend in the Western world is that we are descended from two powerful entities: Father Wolf and Mother Luna. In the time of Father Wolf, the boundary between the physical and spirit world was almost non-existent. Father Wolf patrolled this border territory, keeping spirits on their side and physical beings on the other. 

Some spirits were too ancient and powerful to be destroyed. They were called the Idigam. Father Wolf and Mother Luna imprisoned them in the empty spiritual wasteland of the moon. In the '70s, creatures that appeared to be Idigam reappeared in the world just as an internal war brewed among the non-fundie werewolves. It's how we lost so much territory here. 

There are many theories about why the Idigam returned. Some think it's because the moon landing allowed them to come back with the astronauts. Others think that the Idigam were just forced into hiding. 

Sag'suga Isim is known only in legends. It is a thing of decay that dwells in the darkest places of the Earth. It is never seen, but only acts through agents.