Nightmare Moon has always felt negative emotions, but if there was one thing she didn't expect it was that she; the part from Luna's negative feelings, would feel a positive feeling towards the unicorn who defeated her: love.
Everpony thinks Nightmare Moon is a mare of evil. But in fact, she just wants to be loved. She is looking for somepony who wouldn't judge her and would be friends with her.
Book Two of the Lonelyverse. Having gotten through Harry's first year with a lot of excitement, him and the alicorns thought they could unwind. Sadly, it was not to be. House elves, giant snakes, hidden chambers... Looks like another eventful year!