Something that came to me a while back, that I’ve done nothing with since:
The early twenty-first century.
A few months ago, the question of whether man was alone in the universe was answered.
They came out of nowhere. One moment the skies were clear. The next, large ships were parked above every major city in the world. They ignored all attempts at contact, except to demand surrender. Their demands were ignored in turn.
The conquest was over in a matter of days. Their technologic development was centuries ahead of ours and they had the numbers to put it to good use.
However, there was one thing they didn’t account for in their plans. They cannot be held responsible for this oversight, Humanity itself had long since forgotten about this fact . . . man was not alone on this world.
We were still present, living in concealment, relegated to the realms of folklore, faerie tale, and legend. But we were still here. And so were our cousins and others. And we possessed weapons and defenses that the Invaders had no means to detect.
So, after months of attempting to organize and treaties between various parties, we began to fight back. After all, even though Humanity forgot about us, this world is our home too.
We harass the Invaders when they try to extract the natural resources that drew them to our world. Our cousins sabotage their facilities in the cities. Our allies, untrustworthy as they are, make the Invaders fear the night. And few have yet grasped the true nature of Humanity’s saviors. The collaborators call us terrorists, but what other weapons can we use against a technologically and numerically superior force? Besides, terror is what we know best. We have spent centuries as a source of fear for man. Don’t you use us in stories to frighten children into behaving? Of course you do, even though you claim not to believe in us anymore.
– The world has been invaded by aliens, the militaries of the world proved insufficient to combat the threat
– After a few months of abortive and short-lived human rebellion, the werebeasts and vampires that have lived among humanity for thousands of years took matters into their own hands—werewolves, bears, cats, etc. attack alien sites in the wild, wererats and others infiltrate and attack urban facilities, vampires use their inhuman strength and speed to feed off the aliens.