Back On Familiar Ground

Donnie and I arrived back in Georgia last night. Caught up on sleep after the jet-lag hit me. On the flight back here, we had a conversation on what Donnie has been up to these past few months.

Me: So what exactly happened? Why did you fake your death?
Donnie: It's pretty obvious, isn't it? I had to do it to throw the FBI off my trail. I'm sorry that it ultimately sent them after you.
Me: Off your trail? What were you doing out in the Black Forest? What trail were you following?
Donnie: After seeing all my files, you realize that I knew all about you and Slenderman. I've been researching him for a long time, actually. My father did too, while he was alive. My trail grew cold until I discovered you were a Fighter.
Me: So how did your trail lead you to the Black Forest?
Donnie: After I faked my death, I noticed that your ex was still following you. After you fled the state, she did the same, but in a different way. She booked herself a flight to Germany. I wanted to know what she was up to, and I eventually followed her into the forest. I found the village, and I saw all the proxies there. However, there was one building where a bunch of people, wearing no masks, were being locked up and under supervision.
Me: You mean, there were people there like you that weren't proxies?
Donnie: No, and I planned on trying to free them, or find out what was going to happen to them. The two people that were with me when we escaped were the only ones I managed to save. They chose to remain behind and try to save any others that were brought there.
Me: And what happened to the others?
Donnie: They were brought into the church, one by one. A strange type of mass was held each time. Sam was presiding over each one. I couldn't understand what they were saying, though I think it may have been in Latin. All I know is that at the end of each mass, Slenderman would appear in front of each victim. He would lean over them, and a strange, swirling, black mass would appear between the two of them. He would then lean back, and the black mass would enter the victim's mouth. After that, the person was never the same. He was Hallowed out after that.
Me: Do you have any idea what that black mass was?
Donnie: Not a clue, but whatever it was, that seemed to be the cause of the Hallowing out there in the forest. When you arrived, I couldn't believe my eyes. I never expected you to get there, and once you killed Sam, the masses had stopped for the moment. I'm guessing that maybe He needs an Agent to perform the mass, not just a regular proxy.
Me: So what do we do now?
Donnie: Well, you need to find The Count, still. I'm going to help you. I also have to get in touch with somebody myself. Hopefully, we'll get that all taken care of, then perhaps we can get back to somewhat of a normal life.

So now, I kinda know how this whole Hallowing process occurs. Maybe it was just in this one occurrence, maybe it's in every case, but at least now I have a theory to go on, and perhaps a way to stop it.