User blog comment:Jyggalag Daedric Prince of Order/New Monsters For Future Seasons?/@comment-3045936-20141117082623

Someone else already said. But it's my take.

Hecate: Greek Goddess of Magic.

Now for story elements. Hecate was a god that dealt with magical affairs. She gave her followers magic and taught how to use witchcraft and sorcery. When they died they didn't go to Hades, they become her servent and if they refused or angered her they would be turned into spectral dog (another symbol of her's). After years she off low profile is making deals with people like crossroad demons, but is only offering a month and must die at the end of the month instead of till they died naturally to collect. This is so she can go back to having ten to hundreds of thousands of worshippers.

How to kill. Two methods a bone of a virgin stained in dog blood. Or get her to violate her agreement in a deal. But she can only be summoned by a follower as her true form, is a bright green light of magic and thus not a physical form. Only taking a physical form when she is collecting or making deals.