Somewhere there in the multiverse there is an alien race that desires control over all others. It is an ancient species that has lost more battles than can be counted until it found a solution to most of its problems- humanity. The aliens seed worlds where humans can survive and watch. After humans spread across the globe and wipe out all dangerous forms of life (and unlife) off the planet, the aliens come in, use something along the lines of The Screwfly Solution to remove the remaining humans from their new planet.
The PCs are great heros on their homeworld and have finally defeated the last of the yuan-ti/drow resistance to human dominance. Then everything goes wrong and humans suddenly have mass insanity and kill each other in droves. Will they, and the few others not affected, be able to repel the invaders? Or should they abandon the planet and try to help others?
Though it is sci-fi in feel, I think using ethergaunts or something similar can making it as fantastic as the DM desires.
				
			The PCs are great heros on their homeworld and have finally defeated the last of the yuan-ti/drow resistance to human dominance. Then everything goes wrong and humans suddenly have mass insanity and kill each other in droves. Will they, and the few others not affected, be able to repel the invaders? Or should they abandon the planet and try to help others?
Though it is sci-fi in feel, I think using ethergaunts or something similar can making it as fantastic as the DM desires.