You are a pure soul and as such you are pure light.
As you probably know, the stronger the light, the more it will attrack darkness.
The "person" you saw is pure darkness (you can measure this from a distance when your powers are stronger than his). In that sense, he is not a real human being.
It feeds of fear, anger etc.
When it shows up again, or even already now, it would be good for you to be able to protect yourself and others against the darkness/demons (maybe you already do this).
