Luna's slow descent back to the bitter husk that is Nightmare Moon. In one episode, Twilight and the gang are investigating a series of strange occurrences in the everfree forest. Reports of strange lights and odd sounds coming from the middle of the forest in the dead of night. The mayor asks the main characters to investigate it, since it's disturbing the trio of overreaction ponies (Rose, Daisy and that other one) to the point of hysteria. As they get deeper into the forest, the path that leads them to the disturbance becomes eerily familiar. They spot evidence of a rock-slide, claw marks that have faded with the rains and a bridge that recently has been repaired. It doesn't take them long after that to figure out that the path they're taking is exactly the same one they took to track down Night mare moon. From that point on they're on their guard, remembering what happened last time, as they approach the ruined tower. Instead of the Mare in the Moon they find Luna practicing her powers. When confronted, a shaky experience thanks mainly to Applejack and Rainbow Dash's gung-ho attitudes, they eventually find that she's been trying to regain her former powers to help her sister. She figures that, if she could raise the moon again it would ease some of her burden. The group becomes friends, Luna is accepted as a ponyville resident and Twilight learns the lesson of forgiveness...or something.
Later, several episodes so I guess, an incident happens where Luna forgets to raise the moon. Most of ponyville instantly thinks it's the sign of Nightmare Moon's return, and form a (relatively) angry mob. Eventually Twilight and company manage to stop the mob, but the damage has been done. Luna sees the people of Ponyville for what she believes they truly are: quick to turn on someone they called friend. The darkness in her heart consumes her, and Nightmare Moon makes her triumphant return...