They had been married for three decades before it happened. Alicorns had a longer lifespan, for sure, but they would still die.
So, Pulsar Splash made the journey through the mirror to the other world, "Sunset," he spoke aloud in her now large home, "Sunset Shimmer!"
"Pulsar! What a nice surprise!" Greeted with a pip and a kiss she came down the stairs, "something up?"
"Yeah, we need to go to Equestria."
And so they went and Sunset immediately got the message, they both reported to Canterlot where everypony was already dressed in black.
The guards bowed deeply at them as they both entered, wings entwined. They ascended the towers and entered into Celestia's room first. Twilight, Cadence, and Flurry Heart were all there along with the five elements. Shining Armor was on the balcony, looking at the sunset.
"Pulsar?" Celestia quietly called from her large bed, "Sunset?"
"They say alicorns are always given the choice to hold on when they are passing. I've never seen one ever throw the chance away to have a last goodbye," Pulsar called as they entered and Sunset immediately had tears along with him. The air was depressing
Celestia laughed, "so composed even at a time like this. That was one of the most important lessons you taught us."
"Emotions are as much a tool and weapon as your magic is," the alicorn quoted himself.
She smiled, "so control them and utilize them to your advantage."
"Never in a million years did I think I'd see a student of mine passing away in a bed."
"Humor is the best way to cope, Twilight," Celestia looked at the glaring avatar of friendship.
"Princess Celestia, I...I'm sorry I never stay as your student."
"Sunset, that's okay. I know you truly just needed a different and more special way to grow and I couldn't give you that. You were always an independent learner."
"You live and you learn," Cadence smiled.
"Princess, what...what happens now?" Flurry Heart asked.
"Well, you'll keep ruling the Crystal Empire with Cadence now retired. Twilight. Twilight Sparkle will be the main rulling power over Equestria."
The once student nodded her head in tears.
Pulsar trotted to the balcony while they continued talking to a much older Shining Armor.
"I see why those ponies were mad at you...to have someone you revere and love just die and knowing somepony could prevent it."
"It's hard to wrestle with, hmm? Sunset can offer some advice on that."
"No, it's...it's alright. Hard to believe that they're going away now."
"It's a hard time for all of us...I was considered the twin sisters to be like children to me."
"Now that sounds like you have some stories."
"Shining Armor, have you seen Discord?"
"No, I haven't."
"Someone must tell him, they might not have exactly been close friends but..."
"You need to visit Luna, she said she had some words for you before everypony visits her."
"Thanks," Pulsar gently left the room, closing the door quietly before making his way to Luna's room and gently opening the door.
"Pulsar Splash, I'm glad you've come."
No other pony was there.
"You wanted to see me, Luna?"
"Yes, there's something to be discussed. I've always dreamwalked through your dreams. Every time you had one, Pulsar, I was there."
"I could feel you enter and leave. I appreciate your discretion in regards to my secrecy of earlier years," Luna nodded to his words.
"I...I had this magic for a very long time, and it is unrelated to my cutiemark. I know what the Evermore asks of you and it is a great burden. My sister and I were long standing as a conduit ourselves and it has passed to you permanently, it seems."
"I did not know it reached to you two."
"I...I had a request, Celestia is too proud to ask this so I'm doing it for both of us."
"Of course."
"Bury us under the castle, in that cave at the foot of the Tree of Harmony. That's where everything began and where we made great friends and found our way."
A knock came to the door, "Princess?" It was Twilight.
"One moment, Twilight," she spoke loudly, "another thing, Pulsar. I need you to take this gift, it's a magic I want preserved and can prove very useful as it has for me in my ruling."
Luna sat up from the bed, "come Pulsar," the Prince leaned over and Luna crossed their horns and a magic energy began to flow into Pulsar's body, he knew the traces and signature.
Dream magic.
"L-l-luna..."
She laid back with a slump, "shhh, I need you to have that. I know that Twilight will rule after us, but at the end of everything, you'll still be there. I don't know what the future holds like you do, but you will need to raise the sun and moon and lower them."
"I hope you've been taking notes on the stars, Pulsar Splash."
"I have countless charts," he smiled.
"Come in, Twilight, everypony."
The door opened and the earlier ponies filed in, still crying. The tears had never left Pulsar Splash either.
"When we were there...you were ascending. Dingy Coat told you to be the pony you were meant to be. You'd have made him proud, Master Pulsar," she addressed him as his old title.
"You've made a magical instructor very proud, Princess Luna," he stepped away, giving the other ponies time to speak and say their goodbyes.
It was here that he walked to the balcony and had the thought. Celestia and Luna...their names were of the cosmos and they could control cosmic bodies, so was Twilight Sparkle, Sunset Shimmer, Starlight Glimmer, Sunburst...every mage was tied to the cosmos.
The connections were obvious, magic came from the EverMore...the cosmos. Friendship was the tie to that cosmos.
My child, you understand more than most
He concluded he shouldn't have any trouble moving the celestial bodies, but he wondered what else he could accomplish.
I have given you a very literal name. You will come to know the time to use it eventually.
Pulsar awaited until everypony filed to Celestia except for Flurry Heart. Scootaloo was somehow invited in and she was in Luna's room.
"Luna...is anypony ever ready to die?"
"A funny question," Luna smiled at Pulsar's inquiry, "I don't think so...not until it's the only thing they can do. Then everypony finds a way to cope suddenly."
Pulsar stood at the side of her bed, "I'll walk them for you."
"Thank you, Pulsar."
Luna closed her eyes and her chest in haled before exhaling.
And that was it, the long era of the Twin Sisters came to a formal close.
Canterlot mourned that day and the funeral was a private one, Twilight presided over it as best she could with Cadence and Sunset assisting her. They waited until Pulsar Splash was done making space for their coffins to fit in the rocky cave, transmuting their headstones directly from the rock.
He was staying well composed as there were sobs coming rom outside the cave. Discord, this time, was present.
He couldn't hear the eulogy, and he didn't mind. When he finished, he stepped out of the cave and heard them telling stories.
Twilight saw him, "I-I...it's time to la-," she couldn't get the words out.
Sunset spoke up, "It's time to la-."
"We're telling stories?" Pulsar interrupted and the elements, the alicorns, Shining Armor, Discord, some servants and guards were even present, those closest to the Princesses.
"Before we lay our dear Celestia and Luna to rest, I have a story to tell," his horn lit brightly, "everypony, please come in to the cave," he trotted back in and everypony stood from the transmuted chairs, following him.
Pulsar levitated the decorated coffins in with words from loved ones written on them before setting them on the supports above the holes.
His horn was still lit as he looked at the tree and hit it with his magic, watching a beam fire out from the center and outside before sending directly into the sky at an odd bend.
The elements on it began to shine brightly and color in with the appropriate hues but at the base of the tree, the cutiemarks of Celestia and Luna sparkled brilliantly, casting a yellow and blue light over the whole cave.
"When I was just barely an adult and Equestria has just been formed and the castle was still being built, Celestia and Luna came to me. Starswirl was busy off on adventures mostly. These two asked me to teach them magic and I obliged. I taught them all the understanding they knew, how to cast, how to craft, how to ward and enchant, how to set trigger spells. Everything I knew at the time of fundamentals was passed to them."
"I've seen them grow in ways most ponies would not be able of imagining. I saw their humble beginnings and protected them from malevolent threats they had no knowledge of. As a teacher, I couldn't be more proud of their accomplishments," he felt tears well, "as someone who never knew his own parents, I'm...honored I was something like that to these two. I'll forever remember them as the curious, bright, happy, and strong fillies they always are."
Nopony continued to speak after him, everypony was too busy sobbing, even Discord.
Pulsar sighed, "then."
"It's time to lay you both to rest."
He moved the supports and gently levitated them both down, manipulating the rock again to throughly encase them.
More decades past and they laid Cadence to rest, Shining Armor became sudden friends with Pulsar Splash and used his tutorings as a means of escape until he passed. Twilight took both their deaths hard, she was living quite a while after all. Then she decided to build the friendship gate in Canterlot. It was hard for hime to watch, but Pulsar Splash knew exactly what was happening in front of him.
He was there when the funeral of all the elements in Equestria happened, he was there when the funeral of the elements in the mirror world happened. Sunset took that hard and afterward, her counsel hours in the other world dwindled to just a day a week.
Twilight was an emotional wreck, but she was still putting on a brave face.
"Pulsar Splash! What...what do I do?!" Twilight's voice rang from the entrance of his library while he taught some foals in a tutoring.