After Nightmare Moon was Banished to the moon, The Bat ponies where without a ruler.
The Bat ponies started to rebel against the Sun Princess, and all hell broke lose in Everfree City.

The Sun Goddess didn't had any choice but to banish the Bat Ponies from Equestria so that peace would return.
But she was wrong after the banishment the Bat ponies started to unite and started to fight the Sun goddess and her subjects.

At night they attack, at day they hide.