Fallout 4: The Strange Mystery

There's a case I've always wanted to solve, about the man who does only three things. Appears, kills and disappears. This man appears out of nowhere and helps someone by eliminating a hostile, and it can't even be stopped. I tried, swear in my unreal body, but he slipped right through my fingers. Im trying to find the reason of why he appears to only murder people, he appeard in multiple states and now he's in the Commonwealth doing what he still does, I can't let this slide again. Im gonna catch him no matter what, find out who is he working for and why he kills people, and finally close this case.