Luna had never looked worse for wears. The ghostly night sky mane had faded to a dusty blue, and was in terrible need of brushing. Her belly was swollen from pregnancy, and her tiny feathery son clung to her chest feathers, mewling for milk and warmth. Her face was wet with despair, and she could only do so much to keep herself contained as she trembled.
 
And Nightingale never loved her more.
 
“Nightingale…my beloved friend…I need to ask you a favor…” Luna quivered, “And it’s so wrong of me to ask you…but…I have no other alternative…”
 
“Anything, my Princess.” The young, muscular and precocious stallion stood a few feet from her, keeping a respectful distance. Nightingale was deeply disciplined, but his desire to run over and lavish his lady with kisses and comfort was almost overwhelming. Seeing her feeling such hurt rendered him helpless. He found his heart breaking at the sight of her, though he kept his face emotionless.
 
“It seems that I didn’t think ahead…my son will be presented to Celestia’s subjects tomorrow, and the first question on their minds will be…well, who his father is…” Luna nuzzled her tiny colt, resting her wings weakly on the floor of her room. The firelight danced on her dun feathers, now grown to be tired and graceless.
 
Nightingale studied her. “…what will you tell them?”
 
Luna looked up with a fiery gaze, and Nightingale stiffened, his wings tight to his sides.  
“That is the favor…Nightingale…I-I…I need you to marry me.”
 
That was too much for him. His mouth hung agape, and he broke composure, “Wait…wa…huh?!”
 
“I know, I know! It’s so much to ask for…I just…nopony else would make sense…they’ve seen you with me. They could forgive that. If they knew the truth…it just…it wouldn’t work.”
 
The batpony shook his head. Questions swam in his head, and he picked the wrong one: “So uh…does that mean I get to sleep with you?” As the last word tumbled out of his mouth, his bit his lip almost to bleed, and he felt his face flush with agonizing embarrassment. Did he really just say that to his motherfucking princess? Luna! THE MOTHERFUCKING QUEEN OF THE MOON?!
 
A few moments of hot silence passed. He was shocked to hear the room filled with laughter. A mirthful laughter. Luna wiped a tear of delight from her eye. “Oh goodness Nightingale…where do you come up with this stuff?” She smiled at him warmly. “Come over here…lay with me.”
 
The batpony obediently came to her, his ears folded back submissively. “I sincerely apologize m’lady. I don’t know what’s come over me…”
 
“Oh! Please…with all those winks, flirty remarks, loud bragging with the other guards and apparent flank staring. Oh! And the dreams! My gosh! It’s a miracle I haven’t court-martialed you for such thoughts!” She laughed again until Janus stirred, causing her to nuzzle him back to sleep. She looked back at the stallion with bright, happy eyes.
 
He relaxed under her gaze, and a mischievous streak struck the batpony, “With all due respect, m’lady…you seemed to enjoy those dreams.”
 
She gasped, and playfully cuffed him with her hoof. “Oh! Stop it! The baby’s listening!”
 
The laughed together. A moment ago, Luna was in utter terror of her future. But now…she felt strangely hopeful. She was about to return to her question. Instead, she just looked at her new husband. She knew his answer already, and it was further confirmed by his affectionate nuzzle.
 
It was so kind…so gentle and sweet…she found herself crying with gratitude and something else she thought she’d never feel again. 
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sorry for rly ugly art
 
 
Some context:  
About a month after Janus’ birth, Luna ran into some scrutiny from the public. While they were still unaware of her illegitimate son, Luna knew that when he was introduced, his father would have to be too. Lo and behold Nightingale, Luna’s most trusted guard and first love. (He originally had spots but he’s already super special snowflakey being all brown and stuff, so yeah.) These two had been a long-anticipated couple, after years upon years of tension and flirting that was most unprofessional. To Nightingale’s pleasant surprise, Luna was just as foul as he, and his attraction quickly became a real crush, and then love. Finally, out of curiosity, Luna began entering his dreams and found some very…interesting stuff. But she was intrigued, so she kept coming back…until one day, she was noticed and, well let’s just say he didn’t hold back. Much to his mortification, he found out the next day that that was the real Luna and he refused to talk to her for weeks. Luna found great amusement in all of this, and she grew a special affection for the batpony.  
But then Luna had to leave on her year long trip to the North to research constellations there. What she returned with nobody expected: a baby colt, whom she at first claimed to have adopted. Nightingale, however, noticed the change in his lady and he knew the horror of what that implied. But he knew he loved Luna. And he loved Janus, who fell under his protection. While she was gone, naturally their interaction changed, and it took them until this moment to feel normal again. And the two were together ever since.
 
Another headcanon of mine, though hardly visible here, is that from the point of Janus’ birth, Luna began to slowly lose power and her pelt grew paler and paler. She eventually lost the stars in her mane, and its ethereal appearance. And eventually, she would age as a normal pony.