Theres A bug in ponyville choosing a pony to infect with its deadly virus that turns the chosen ponies eyes pupils FULLY red and turning the sclera BLACK. (Sclera is the white in your eye)
It also gived the chosen pony some sort of power when a ponies mouth opens, red, bloody tentacles come out of the chosen ponies mouth and into the other unlucky pony, draining most blood and leaves them with little power, Little strength and full, black eyes, and those ponies.... Those ponies soon die. Pony magic is to weak to defeat the bug, but the power of 2 alicorns is enough to send the bug away for 500,000 years.

But, What happens when the chosen pony is lunas sister.....