Here, we have a what-if/alternate universe situation:
Nightmare Moon is a creature whose magic is powered by fear (sorta like Sombra), so she goes about wreaking havoc and scaring the ponies of Equestria. One day, Luna finds a young orphan unicorn (Twilight) and raises her as her own. She warns Twilight the world is dangerous, and to beware of Nightmare Moon. Over the years, Luna becomes addicted to the power she has as Nightmare Moon and starts spending less time with her adoptive daughter. Feeling lonely, Twilight ventures out and eventually meets Celestia. Celestia provides the love and care that Twilight misses from her own mother, and soon sees Celestia as a new parental figure. Then, one fateful day, Twilight finds out that the very monster she was warned about all those years is her own mother! Twilight is obviously upset about this. Luna blames Celestia for "stealing" Twilight away from her, and decides to go full-blown Nightmare Moon forever.
The story is loosely based on the plot for the movie "Repo! The Genetic Opera."
The song is "Let the Monster Rise," sung by Alexa Vega and Anthony Head "Repo! The Genetic Opera" belongs to Twisted Pictures/Lionsgate "My Little Pony: Friendship is Magic" belongs to Hasbro/DHX Media