"I dwell in your dreams; I savor your nightmares. Now you will serve me." ~Vaermina, Prince of Nightmares
"I am the princess of the night, thus it is my duty to come into your dreams." ~Luna, Princess of Equestria

Dreams are a powerful thing; there's a special sort of magic to be found in the realm of dreams, one that holds infinite potential. A beautiful dream emits a radiant light that comforts the one who conjures it.
A nightmare, on the other hand, holds dark magic that burdens the sleeper's mind, crushing them under its influence.
For a being who can feed on that magic, nightmares are a valuable source of sustenance. To a princess who watches over dreams, they can be a burden. When the one who feeds on dark thoughts turns its eyes toward Equestria, it falls to the guardian of the night to stop it.