When Luna was banished, it left a hole in Equestria's defenses. While Celestia could care for both the sun and the moon, watching over Equestria's dreams was beyond her power.

For a thousand years, the Sandmare has been a regular unicorn, trying her best to ensure that little ponies sleep peacefully. But it's a lonely life, one that often ends in madness or early death. And over twenty years ago, the last Sandmare was brutally murdered by a monster, who is determined to topple Equestria.

Moon Shadow has no idea of any of this. She's the current Sandmare, living in Canterlot and wishing that she could have a normal life, like everypony else. But when stars begin to move on their own and past evils re-emerge, she's got no choice but to live up to her very unwanted destiny.