As Luna's horn began to glow a sense of unease suddenly filled Dark. He looked around and realized he was on a pitch black void of darkness where it looked light light never touched it. He could see in the dark, so that was not a problem, but he looked around and saw a door in front of him.
Princess Luna suddenly stood next to him. "Dark, behind this door is my other side," she began. Her horn glowed and revealed how chained up and practically nailed shut this door was, it was clear that this door was designed to keep something, anything out but also seemed to keep something in. As Dark approached the door he noticed that the lock was bizarre, as on the bottom of it was shaped like all of his other friends cutie marks. He turned to Luna and was about to say something when Luna answered for him. "When I was blasted with the Elements of Harmony this door was created by them. It can only be opened by one of two ways, either by myself for a brief window or by the former wielders of the Elements of Harmony if they wish to make this permanent." She looked at Dark. "Are you absolutely sure you want to do this Dark? I value our friendship and I'd be perfectly happy if you and I can just remain friends with each other. I would not think any less of you if you wished to have this view of me of just a hero," Luna proposed, her voice sounding like it wasn't what she truly wanted.
Dark shook his head no. "I have to know if I am in love with you, or if I am just going to be friends with you." He smiled at her and continued. "I can't just not know Princess Luna, because if I am in love with you wouldn't that be a good thing?" Princess Luna paused looking in surprise at that as a tint of pink filled her cheeks. "What ever is behind that door, know that you will never ever stop being my hero."
Princess Luna nodded and her horn glowed. The door itself glowed with an ethereal light as it slowly crept open, and Princess Luna motioned and Dark walked through it.
"Good luck, please be true to your word." Princess Luna's voice was almost inaudible as Dark trotted through.
As Dark ventured forward. He looked around and muttered, "What a bizarre room." The room was nothing but a blank white void, there was nothing, nothing there, and even though it was a dream Dark could feel how cold it was. He looked around and then saw that as the door closed a shadow clung to the door. The Shadow moved and darted around as he looked around frantically trying to locate where it was in relation to him. Suddenly the room twisted and distorted as Dark saw the trees of the Everfree Forest sprouting up from out of nowhere, he looked around and then saw a large castle sprouting from the ground. Now in a panic he backed up only to see that the castle doors opened and the shadows wrapped around his leg and gripped them, pulling him into the castle, the door slamming behind him.
Dark looked up and saw a familiar silhouette. "Princess Luna," he whispered with a sigh of relief. "If this is just your dark side, I can take it." Suddenly, he heard an echo of her voice, but it sounded twisted and distorted like it was mimicking her. The figure walked forward revealing while yes, it has her mane, this was not Princess Luna, this thing was pitch black fur. It's eyes Dark has only seen them once when Princess Luna was angered during the Princess gathering.
The thing smiled wickedly. "Welcome, Sidewinder, I've been expecting you." It floated down and landed next to him. "I wonder, did my weaker half ever give you my name?" Dark's muscles tightened as he squared his jaw. "Oh that won't do you any good Sidewinder, but I take it she did not." The figure laughed. "My name, is Nightmare Moon!" Dark paused, in shock, but he really shouldn't be surprised. Princess Luna has told him several times before that she became Nightmare Moon out of her jealously. "So, my weaker half wishes to test your love," the infamous Mare in the Moon mocked as she walked next to him. " How pathetic, she should know that we can never ever fall in love. Only the weak minded would dare think that we could ever be in love with any pony, not only that but you?"
"What's wrong with me?" Dark responded, his voice trembling.
Nightmare Moon started laughing maniacally. "What's wrong with you, let us count the ways, shall we? You're an obsessive nutcase who worships our weaker side blindly while trying to ignore what she has done. You get called a hero for actions that she did, not you and deep down you know that all she see's you as is a creepy weirdo with no sense of taste, no strength, and a pathetic guard that she gave the title of knight out of pity, not gratefulness." She smirked. "How many times did you come into last place during your training? Several right? Oh I remember it well, you crying to us begging us to give you an answer why, while we pulled something out of our flank to make you smile, while secretly, we were disappointed in you, because you were so weak and pathetic." Dark looked away from her trying not to give her the satisfaction that he was listening to everything that she was saying. "Oh, and then there was mother."
Dark paused at that. "The Queen of Dreams was crazy, she almost killed me!"
"It was still her mother boy, still her mother you helped defeat. You struck her, her mother! How could she ever love somepony like you? Somepony who broke her mother? I mean look at you, you're a monster, you always were and you always will be!" Nightmare Moon taunted. "Please, tell me that you still love her, or can even be friends with her, because of that!"
Dark was silent for a while as Nightmare Moon began laughing almost in triumph, her laughter echoing through her mind as he thought to himself that she was right about him, all the things she was saying, all the things Nightmare Moon said taunting him, berating him, and humiliating him. It was all the truth.
There was silence before Nightmare Moon asked, "Well? Do you still love her, after meeting me, the one who tells you the truth?"
Dark paused as there was still silence in his mind. He tried so hard to think about it, he found no way to respond to the things she said. In his mind memories of Princess Luna and him talking replayed in his mind over and over, and over, until eventually Dark finally spoke. "You're right, it doesn't make any sense."
Nightmare Moon smirked and stood up straighter. "I knew it! You don't love her, besides, even if you did, she's immortal and you are not, it would never have worked out!"
"Shut up!" Dark finally exclaimed, venom coating his voice. "It doesn't make any sense, but you know what, love never does!" Nightmare Moon's eyes widened as Dark spoke to her, a clear air of defiance in his voice. "Princess Luna means the world to me and I love her! Meeting you have only doubled that in size as I know that she is nothing like you!" Dark glared at the alicorn before him. "So what if I'm not the strongest pony, so what if I'm not immortal like her? I love her! You can throw as many insults and put downs as you want, you can show all the ways it doesn't make any sense, but Princess Luna means everything to me and I can not let someone like you ruin it!" Suddenly a light surrounded Dark as he walked forward to confront Nightmare Moon, when suddenly Dark stared face to face with Princess Luna herself.
Luna's eyes were running. "Dark." Dark was about to say something when she pulled him to her. "Thank-you."
"Princess Luna," Dark whispered as they looked each other in the eyes. "I love you."
"I know," Luna replied, a tearful smile on her face. She lent down and kissed her forehead, a brilliant light culminating from that action. Dark looked face to face with Luna back in his own dream world. "Dark, when you get back to Canterlot, I have to tell you something personally, I can't do it in a dream, it's just not proper." Dark paused and nodded. "Now, complete your duty, so we may speak." She started turning around and stopped. "And Dark," Dark looked at her and she smiled. "Thank-you again."
Dark's eyes opened as he saw that it was the next day, and a new warmth suddenly flooded his body over the dream he just had about Princess Luna. He smiled as he stood going to find Fluttershy, the two of them were going to hang out with Queen Chrysalis today, per her request believe it or not.