The story of 'The Mare that eats the Monsters' is an old, but still very well known Equestrian campfire and bed time story, often told to scare little fillies and colts into behaving themselves. It tells of a mare who loves Equestria and the ponies who live in it despite how frighteningly different she is. She protects ponies across Equestria from nightmarish creatures and monsters that would seek to kill ponies wherever possible.
What most ponies don't realize, but what Twilight Sparkle is soon to learn, is that the mare of the story was and is very real.

And the stakes of her mission have never been higher.

(This story has the horror tag, yes, but it isn't the dominant theme here.)

Things didn't go, as planned, in the Crystal Empire. Sombra was defeated, but he used a clone to try and take the Crystal Heart. Now, he has Twilight Sparkle captive, and trying to force her to become his student. Celestia, stricken with grief, sends every pony she has to search for her valued student, who was like her daughter... Not knowing, another has come to help. A being that has been around far longer than any other. One that was old when Celestia was but a foal, and has sensed the trouble within Equestria. Knowing he could not physically help, the being decided to assist Twilight in learning a power that many consider dangerous... But it is, truly, the only way to defeat Sombra.