The war begins with Nighthawk finding out that the Darkness has began raiding Winter Village, a military operating base for the Crystal Empire and if taken, could result in destruction to the Empire. Equestria makes their move to defend the village, but will soon realize that they will need some help. And that's where an unexpected guardian comes to the rescue.
(Special thanks to Cinnabar for helping me out: https://www.fimfiction.net/user/307472/CinnabarPony)