http://y5wnzw4e6i7srm2gqadlow5anhlaj5avdkzbwzbmrxwkygxdp7ffieqd.onion/blog/alien-threat-awareness.html
Their civilization will begin to collapse """for some reason""" - food shortages, pandemics, violence, if you can name it - its happening. The point of this is simple: make them look for a savior as billions of them die, removing a huge useless chunk of the population... leaving the lucky (NPCs who did not die), strong, and wicked.