I loved Nightmare Rarity, and having it be where Nightmare Moon was the Pony World's equivalent to Marvel Comic's Venom was genius, because it doesn't limit Nightmare Moon to being simply the Evil Side of Luna, anyone who is possessed by this "Nightmare Symbiote" could become the new Nightmare Moon, and having it be where the New Nightmare Moon was 1 of the Mane 6 only made it all the more meaningful, because our heroes not only had to face off against a new foe(or the return of an old foe depending on how you look at it), but they ultimately ended up having to face off against one of their own, a band of friends having to defeat their friend turned evil(or at least possessed by a living force that turned her evil), that's just one of the many reasons why Nightmare Rarity was such a great character, she was both a Friend and a Foe rolled into one.