What Luna and Megan didn't know was that Celestia was sort of eavesdropping on them. At first, she got up and ignored what she heard while going into the kitchen to get some cake due to feeling the urge for a midnight snack. As she passed Luna's room again, she decided to enter and give her take on what happened when Luna was Nightmare Moon and explain to Megan what happened from her side.
"I can see that my sister brought you over to tell you about her dark side." said Celestia.
"Was she not supposed to do this?" replied Megan.
Luna then said, "I'm very sorry sister for telling her this when it's supposed to be a secret to most."
Celestia responded with, "That's alright Luna. Since Megan was Ponyland's savior, she was going to know about this sooner or later."
Megan then asked Celestia, "What was it like to face your sister when she was in that form?"
Celestia starting to feel sad said, "I wasn't just sad seeing my sister that way, but also scared having to actually fight her when I really didn't want to."
Now feeling curious, Megan asked, "Do you have an idea why did she become Nightmare Moon?"
Celestia's reply came as, "It was sort of my fault that she wound up that way. If I had devoted some of my time to her while performing my duties as the ruler of Equestria, I could have probably stopped this from happening altogether. However, I just left her alone most of the time, and though she might have been going through a phase in that interfering would just make it worse, which is why I chose to stay out of it originally.
Megan then said, "What about her song, The Moon Rises? Did you ever hear it at certain points and thought it could have meant something?"
Celestia answered with, "At first, I thought that Luna was just venting her anger when I heard that song especially when she referred to me as Sister Sunny Day or even to make the claim that she isn't the lesser light. When I heard her singing that song more times, I noticed more words added such as claiming that it will be the final setting of the sun and even the mentioning of eternal lullabies, but I couldn't put my hoof to what she really meant when she sang that. However, when I did, I was too late and she became Nightmare Moon."
"Nowadays, I don't sing that song anymore, because it only made me feel worse and made me refuse to lower moon for real when I really started to feel lonely, neglected, and even furious." responded Luna.
Megan then said, "There was a time I sort of felt that way when it seemed as if I was no longer needed to come back to what was then Ponyland and had to resume my normal life as if coming her was an escape from reality."
Celestia continued the conversation with, "Getting back to what happened, even though she tried to kill me, I just couldn't kill her back. There was even a point where my horn glowed so much that one blast would have done the job, but I just couldn't do it especially because she was still my sister despite that. Instead, I just did the next best thing and used the Elements of Harmony to banish her to the Moon, which resulted in turning my mane to way you see it now as a side effect to that."
Megan then asked, "Did you miss her even after all of that?"
Celestia answered back with, "Yes I did. I even sang a special lullaby that in disguise was my confession to what she became of. However, just like your return to Equestria, I was still hoping that I would be able to see her again and she would be back to herself."
Luna then said, "When I was on the Moon, I had a lullaby of my own, but this one made me feel both neglected and angry. In a way, I did want to have my revenge for what happened, but I also wanted the love and respect I didn't have originally."
Megan then asked, "Who was in charge of making the Moon both rise and set while she was banished there?"
Celestia then answered, "I pretty much did it myself even though it's normally not my ability to do so, but there is a possibility I did it with a spell to make my sister work with me so that it could work that way while she was there."
After hearing all of that Megan said, "Princess Luna, I can understand what it's like to be someone like you, but you shouldn't always allow for that to get the better of you."
Luna then said, "Maybe Celestia did care about me after deciding to release me from the Moon?"
Celestia replied with, "For some reason, I don't recall releasing you at any point."
Feeling puzzled, Megan asked, "If you didn't release her, then who did?"
Luna responded with, "I guess we'll never know who really released me. but I do know that when I was released, I found myself over at town hall in Ponyville introducing myself to all the ponies there and striking fear to all of them."
Celestia then said, "Nonetheless, Twilight and her friends used the Elements of Harmony to help change her back, and I was willing to accept her forgiveness despite what she did."
Megan said, "That's very touching to hear about how after what you two went through, you managed to get back together again."
Luna then said, "They could have at least helped me return to myself without taking off so many centuries of my life and making me end up as a foal."
Celestia then responded with, "I'm sorry that they overdid it, but the Elements of Harmony were still new to them, plus you should be glad that I found a spell that allowed for your current age to be restored even if it meant you had to wait some time for it to fully work and not leave the castle until it was done."
Megan then asked, "Why was having this dark side kept a secret?"
Celestia then answered, "I felt that it would be for the better if fewer knew, and I did that for Luna's sake."
Megan replied with, "Deep down inside, she doesn't feel very comfortable with this and she even told me how all those other things would have been better for her than having to live with that secret."
Celestia then mentioned, "First off all, I did overhear all of that, and I would never do any of those things to her no matter what the cause was. Secondly, Luna isn't the only one here with a dark side, because I have one as well. When Starlight Glimmer had me switch roles with my sister, I saw my dark side known as Daybreaker in the dream realm. Unlike Nightmare Moon, this one can form if I can get express a high amount of rage with my mane and tail turning into a fire. Fortunately for me, there hasn't been anything that can make me mad enough to become that. If I did and Twilight and her friends would help me change back to myself again, I also would have aske my sister for forgiveness as well."
Megan then thought, "Maybe if Princess Luna had a good amount of friends by her side, she might just be able to apologize and ask for forgiveness for all the actions she did as Nightmare Moon and not feel feared in doing so."
"Do you think it will work?" asked Celestia.
"I hope you're right." replied Luna.
Celestia then said, "Now that we have settled this, let's all go back to sleep and have you go back home in the morning."
Celestia went back to her room to sleep for the rest of the night while Megan was given the guest's room for her to sleep while saying good night to Luna as they both left her room.