It is impossible
Nightmare Moon was laying in a more or less intact room of the Everfree Castle. She had created herself a body out of her own magic, years after being defeated by the Elements upon her return from her exile. Even though she was bested, the Elements of Harmony did something good to her. She was no longer another spirit trapped in Luna's body. Yet, as her own being now, she didn't have anything to do. She contemplated life, choices, events, motivations, justifications.
She wanted to kill Celestia, take over Equestria's throne and plunge the world into eternal darkness. She wanted to take the Sun from the ponies of Equestria. The life giving sunlight. The crops would have withered, the planet would have become a frozen wasteland, where she would be the only one alive. Yet she hated loneliness. Her plan wasn't well thought out.
The room she settled into was in the east wing of the once great and majestic castle, somehow intact, apart from being completely empty. Sunlight flooded the room through lare holes in the walls, obviously windows, but without glass panes in them. Nightmare stood up and moved to a part of the room where the sun warmed the floor. She laid down and closed her eyes as the warmth seeped into her black fur. The mare let out a contented sigh at the soothing sensation and rested her head on her front hooves as she let sleep take over. She loved sleeping in the sunlight. It warmed her, engulfed her as a comforting embrace... Her embrace...
She woke up on a hilltop outside of Canterlot. As she laid on her stomach, the grass brushed against her legs and torso. She was content with the softness of the grass and the warmth of the Sun. She heard the rustling of grass as somepony was approaching, and moments later she saw a horn, a flowing, multicolored mane, a beautiful face with a gentle expression over it as an alicorn with a perfectly white coat came closer to her. Neither of them spoke. They didn't have to.
Nightmare raised her upper body to a sitting position and wrapped her front hooves around Celestia's neck, nuzzling into her chest, rubbing her face on her soft fur. Celestia sat down as well, and brought her hooves around her back, slowly rubbing it up and down. They embraced each other for minutes, then Celestia dragged her hoof up to the back of Moon's head, gently stroking her mane along her neck as she did so. She then slid her hoof to Nightmare's face, caressing her cheek with the tip of her hoof. Moon hummed with contentment, feeling the caring touch on her face.
Celestia gently pushed her onto her back, and as Nightmare let go of her neck, she laid on top of her, resting her head on the dark mare's chest. Her wavy mane has gotten over Nightmare's face, who didn't mind it at all, instead she let it cover her face with its ever-changing colors as she inhaled its scent of sweet lilac. She lost herself in the ocean of waving greens, ceruleans and pinks, not thinking of anything in particular hugging Celestia to her chest with a clouded mind. Minutes passed in this peacefulness, until Celestia pushed herself up and stared deeply into Moon's cat-like eyes. She smiled with a loving expression, and so did Nightmare.
Celestia gently swept Nightmare's starry mane out of her face, keeping her hoof at the side of her face. She slowly inched her muzzle closer with half-lidded eyes, and their lips touched. It wasn't a really deep kiss, they just locked lips, feeling the love course through each of them as Nightmare put her front hooves to Celestia's side, caressing them, whilst Celestia tenderly held the sides of Nightmare's head. Nightmare loved the warmth of Celestia's lips being pressed to hers, flooding her with warmth and the relaxing feeling of being loved. Then Celestia broke the kiss. Her eyes were glistening with tears, some starting their journey down her face, leaving stripe-like stains in their way.
The world started to lose its perfect warmth around Nightmare as she stared into Celestia's trembling eyes, who surprisingly still bore the gentle, loving expression, not caring about the tears flowing down her face. Nightmare felt heavy and cold as Celestia stood up and smiled at her before leaving. She let her head fall back down into the grass, which was no longer relaxingly brushing against her fur, nor was it providing a soft place to lay upon. It became cold and hard under her back.
Then her eyes opened. The scent of lilacs still lingered in her nostrils as she looked out the window-hole on the wall. She estimated it to not to be much after midnight. Only when she wanted to get up did she realize she was laying on her back. Nightmare rolled over and stood up. She did not understand anything. She didn't understand why she was feeling empty, or why did she feel like crying. It was just a dream. A dream that is not real, nor can it ever become reality.
She wouldn't love her anyway. It would be impossible. She was a monster, nothing more. A monster who took her sister away from her for a thousand years. A monster who wanted to take the throne, perhaps her life as well, plunging the world into eternal darkness with no sun to provide warmth, joy and life. But that monster was a thing of the past. The monster yearned for the comfortingly warm embrace of the Sun's light. The monster loved the day. Loved the Sun. But the Sun would not, and could not accept her darkness. She did too much bad.
Nightmare left the room that was once Celestia's bedchamber. The only intact part of the Everfree Castle. The favorite place of the Night. A single tear slid down her cheek as she leapt into the air and unfurled her large, black feathered wings. She knew that more would come. She felt it in her heart.