User blog comment:Zathronas/Analysis by an author: Volume 2 episode 9: Search and Destroy/@comment-10390252-20141003134556/@comment-24218038-20141003140158

You have some great speculations and imagination but I haven't seen anything in the show about either a cult or a dark aura.

In fact, they have told us several times that the Grimm do not have any aura.

I'm not saying you're wrong, well..I kinda do but here's my reason.

Speculation has to be based on what you see in the show or else we're treading fan fiction territory which is wishful thinking more than not. It's not a bad thing but it doesn't really fit with what we've seen.

Still,the cult thing could be possible.