oh, are you talking about this guy?
i've encountered him before, i've made a post about him but it's quite buried by now so i might as well give a bit of detail on him here
different ambience starts playing when he spawns in, which is used in portal 1 and possibly half life 2 in ravenholm as well
this guy follows you in a straight line no matter where you are, it'll fly or phase through solid objects to get to you as well, he even speeds up exponentially if you are far away from him
all ways of recording it, like the digital camera or viewing it through cameras overlays a "missing texture" over the image, it's a missing texture asset used in source games
when the entity touches you, the game freezes and makes this grid of weird lines before causing a "fatal error!" message to show up and crashing the game
Viewing post in 0.8 discoveries (SPOILERS)
I was lucky when I encountered him that he killed me quickly, before the game autosaved. He spawned just a few feet away from me at a sat while I was dealing with a pesky mannequin. Turned around, he was moving toward me, and then I got the lines and crashed.
Fortunately, my previous save was just before I came down the steps at the sat to find Manny creeping on me, and when I loaded it, the Shadow did not spawn and I just continued playing. I would be super cranky to have it save during a hunt and be unable to continue that particular game run, because it does not look like he eventually gives up and you can continue with your work/exploration.