This kind of reminds me of the Twilight Realm from The Legend of Zelda: Twilight Princess. Sad, lonely, and every sound is emphasized because of the vast emptiness and lifelessness that exists there... the only 'life' are the Twili people and the Twili beasts and other corrupted creatures that exist. Most living creatures would become spirits or be corrupted by the Twilight. Nightmare Moon would be like Wolf Link, a lonely creature that cannot be seen by the spirit people (or ponies in the case of NMM) but can be seen by the corrupted beasts. She is on her own.