Darkest nightmare night nr: 24.
 
A pair of unexpected visitors enter Ponyville, on this night of all night. To those unfamiliar with its costumes, the sight must be quite shocking.  
(Especially if one has the mindset of ancient ways.)
 
 
Stygian: “Sir. I believe Its a festival dedicated to the celebration of your darker side. Its inspired by the time Princess Luna turned into Nightmare moon.
 
Starswirl. “Nonsense! The filly I trained as my pupil would never sucumb to Evil influences.”  
Stygian: “It wasnot external forces sir. It was jealousy and other things that festered in her mind. Did you read the books on modern psycology? They theorise that all ponies have darkness inside them that needs to be adressed.”
 
Starswirl: “Such hogwash! All knows that the only thing to do with evil is to banish it, from the body and soul.”
 
Stygian: “ It’s in the history books sir. Princess Celestia had to banish Nightmare moon …”
 
Starswirl “Aahh. Its good to know she took after my teachings.”
 
Stygian Sigh: “Of course she did, sir.”
 
(note: I like to believe that Starswirl way of looking at psychology, though it was the top of philosophy in his age, are actually a very unhealthy viewpoint that forces people to hide or suppress their thoughts without addressing the underlying issues, less they be accused of being “Evil” and getting misstreated for it.  
Sort of like how our medieval way of looking at the mentaly unwell was to believe they were posessed.)  
A very “Hear no evil. Speak no evil. See no evil = there is no evil” mentality.
 
(Note 2 : bouth luna and celestia would have taken after that philosophy, and we know how that turned out.)