There's a type of being my world, called a Noah. They're extremely powerful, and the source of their power is dark energy - like the malevolence that Hellions use. There used to be a belief among Exorcists that Noahs are immortal, because of how strong they are.

The thing about Noahs though is that there's only 14 of them in the world, and they use humans as hosts. When one passes away, they can implant their memories into another host and they'll eventually take over that host, eroding the original person in the process.

That shadow, it's a sign that you're a host.