When the evil Grogar returns to bring eternal darkness & decay to Equestria and beyond, only 1 bad-ass anti-hero(who's the last of his kind) has the power to stop him, and Princess Twilight wants to show this anti-hero the true power of friendship.
Luna visits the Whitetail Wood, to beg forgiveness of a lost tribe of night ponies cursed by Nightmare Moon, and finds that she has wronged them worse than even she thought possible.