WARNING: SPOILERS
Well, it's one of the "Secret !FUN!" from the "Misc. Features" in the launcher.
Basically, it's a person hidden in your population, that gives a bit of warning, then he starts to wreack your population from a distance.
Your only way of finding him is with a Ward of Armok, and quarantine your population near it (or do it bits by bits).
Prepare your army, and also prepare in case it happens to be your strongest, best suited soldier (just because it can happen).
That's mostly what I know, because I personally disable the hidden stuff after a while in the early days.
Anyway, I hope it helps a bit.