Your character only knows what they've seen.
Ergo- You know Steve Foster is the one who's recruited you, you are working for him and whatever gov't organization he belongs to(unknown to you, assume federal law enforcement)
You also know about the Blackriver Killer from Steve Foster and that news about such has somehow failed to reach across state lines to big news companies.
-The Blackriver Killer is active only in Blackriver County
-The Blackriver Killer targets those with the same black cauliflower-like welts on their bodies
-Your mission here is to investigate and apprehend the Blackriver Killer
-The FBI is not butting into the case so far
-The CDC's team sent to Blackriver County has seemingly disappeared
-That is it