This is my take on the Great Invasion event. I've modified the timeline and dialogue for narrative purposes.
To skip straight to the smut:
Chapter 2 - Mutsunokami/Sen
Chapter 3 - Kashuu/Rinko
In the Middle of a Fire while Standing in the Water
Chapter One
Kashuu brushed the final stroke of nail polish on Mutsunokami's thumbnail; his right one was red and his left one was orange.
"Well, look at that." Mutsunokami raised his hands to his face. "I can't decide which one looks better."
"When you do, let me know. I can paint your other nails." Kashuu screwed his nail polish's caps closed.
Mutsunokami blew air on one nail and then the other. Midare, Kotegiri, and Jiroutachi had opinions about which colour was best. He could have one hand's fingernails painted red, and the other in orange. He snickered at the thought; why chose only one?
Kashuu propped his elbow on the kotatsu. A faint crescent moon peered through the clouds and dimming sunlight. The sages were still talking to each other. How severe was the anomaly in the Mino, Aonohara era? Usually, other swords entertained guests from the Ueda Citadel; Azuki, Shokudaikiri, and Hyuuga didn't have time to prepare baked goods or umeboshi as they and their teams were out training.
"Kashuu, Mutsunokami." Mikazuki stood by the room's shoji. His armour and white and navy clothes were pristine; one couldn't tell he returned from the battlefield.
"Welcome back." Kashuu straightened.
"Yo, how's it goin'? Has anyone told you how many enemies are out there?" Mutsunokami chuckled nervously. "I'd love you to tell me the numbers are a joke..."
Mikazuki stared at that small moon in the sky. Soon, it'd grow into its rounded, fullest form. Even when the moon was fully exposed, it was mysterious and powerful. It tugged at the earth and its waters so far away.
"It's a Great Invasion," Kashuu said. "Master mentioned that the scale of it is expanding more and more."
Every sword knew about the hoard of enemies arriving soon. They always had an infinite number of enemies to take down. Why were things different now?
"Right now, we have no choice but to follow the orders of the Government." Mikazuki turned around. The red and yellow tassels in his hair, gold armour, and blue sleeves swerved with his movement. "If one makes an error of judgment, one may end up unable to protect even the things that must be protected."
"Huh?" Mutsunokami sat upright. He failed once, centuries ago, to guard the most precious thing to him. He lie on the floor and watched assailants slay Ryōma. When he reached out to his sword for one last grab at survival, Mutsunokami remained sheathed, unused. Did Mikazuki know something about the Great Invasion?
The tachi stepped into the hallway, whispering about the stars and the sky. His chuckle echoed on the walls.
"Don't think too much on it." Kashuu stood and smoothed his hakama's wrinkles. "That old man's from the Heian era; they spoke in stanzas back then. Should we check on our masters? You can show them your nails."
In the centre of a courtyard, under a torii gate, was the time travelling device. Rinko and Kashuu headed to the nearby engawa expecting a team to return. They had to be rearranged again; each sword, both on and off the teams, needed to train for the incoming invasion. Kashuu held the lists in his hand.
Mikazuki sat on the engawa alone, staring at the torii gate.
"What are you doing?" Kashuu stepped ahead of the sage to speak to Mikazuki.
"Ah, I was just thinking about getting some tea. Would you like to join me?"
Kashuu couldn't risk staining his teeth. There was plenty of matcha in the pantry. Uguisumaru could brew the drink for him, as he had done before.
"Did you come to see the moon's true colours as well?"
It was morning. The clear blue sky had no satellites in sight. What was he really talking about?
The torii gate glowed gold; a team was returning.
"Excuse us, Mikazuki, we have other things to do."
Rinko stepped into the courtyard and turned back to look at Kashuu. It was his cue to follow her.
"...I'm the same as you. I just wish to protect." The crescents in his eyes gleamed gold.
"Protecting history. That's the duty of us Touken Danshi." Kashuu's master would keep him forever, so long as he remained pretty and useful to her; her goals were his goals. Every single sword understood that. Why bother mentioning it now?
"It's very reassuring to hear that from you." Mikazuki chuckled slowly.
The torii gate's light flickered and a team appeared before the saniwa in a dissipating cloud of light: Ookanehira, Juzumaru, Daihannya, Hachisuka, Urashima, Hasebe, and their black Konnosuke.
Kashuu walked towards them to provide them with updates.
Sen replayed the call on her laptop for Mutsunokami. A man with black and blue hair wearing a fox mask and white gloves spoke.
"There is corrosion on the jumproute to the headquarters. The Government is currently entering emergency defense mode."
That was the final correspondence from the Time Government. In the last virtual meeting with officials, they mentioned fortifying their facilities without providing further details. They had to ensure their own preservation. Why couldn't they be straightforward? What were the citadels supposed to do during this time? They didn't have the materials to raise walls or dig moats. All they had were their own lives to resist the revisionists.
Was the Time Government abandoning the Ueda and Ashina households to preserve themselves or were they deliberately trying to attract the enemy to the sages? Either way, Sen and Rinko had to plan.
"Survive." The video blurred into greyness, marking its end.
We are not expendable. Sen gripped her red hakama. They wouldn't fail. She wouldn't allow anyone to die. Whether it was the Time Government or revisionists that came for them, the citadels would vanquish their enemies.
"Sen-chan." Mutsunokami hugged her from behind. His hand rubbed her fists. With his callouses on her soft skin, her grip loosened. "You're always doin' your best. We trust you."
The fact that there was so much at stake, including their lives, made her role all the more critical. Every time she penned a list to form a team, she sent them on a mission they may never return from. They prepared, but giving them equipment and omamori weren't safeguards from death. Injuries on the battlefield, from a minor scrape to something major, were reminders; swords manifested for this war and could perish.
Sen turned her head to face Mutsunokami. His brown bangs brushed against her forehead. He kissed her cheek and nuzzled his face on her neck. He had been with her since she started as a saniwa; despite the burden, she cared for everyone in her citadel. "I wouldn't want anyone else lookin' after us, Sen-chan."
"Mu-chan..." Sen stroked his forearm. Past the bandages, his elbow was smooth. With all the future fighting, it was impossible to know when they'd have their next intimate moment. She parted her lips to ask for a kiss.
Her phone shrieked an alarm and flickered red. Someone, something was approaching the citadel.
"The heck's this?" Mutsunokami blurted.
The earth quaked and then flashed white. The laptop screen went black and the phone silenced. Through the outer shoji wall, Sen's bedroom was illuminated in white light. The night could never be so bright; it was meant to be dark and peaceful.
Sen pushed herself to stand, but was restrained by Mutsunokami's arms around her waist.
"Let me check, Sen-chan." Who knew what the enemy planned out there?
Her brown eyes pleaded with him not to sacrifice himself for her. In the end, Mutsunokami had the strength to hold her back. He kissed her lightly on the lips. She'd give everything to ensure everyone else's safety, but Mutsunokami had to ensure she survived too; she had a long life to live. He'd stay with her as long as the gods allowed it. He wouldn't fail protecting his master again. When Sen slumped back against him, he stood and headed to the shoji. His shadow extended along the tatami mat as he stepped.
What did this light outside mean? When Mikazuki spoke in riddles, was he dropping clues?
When he slammed open the shoji, the room was beamed in moonlight.
Based on what he saw and read about the universe, the existence of a place like this was impossible. The moon, a moon, was on the horizon. Its craters were huge and visible as it rotated and its light reflected off all the sheets of ice and water between it and the citadel.
"This scenery...," Mutsunokami whispered. "Where's Mikazuki?"
"Master!" Konnosuke called from the hallway. "The Great Invasion is nigh!"
The last time the Ashina and Ueda households were together, it was to celebrate the second anniversary of the sages' inauguration. Every sword sat seiza in an audience hall. With so many additions, all the swords gathered in a courtyard while draped in moonlight.
They should thank the gods that the citadels landed in this world beside each other.
Mutsunokami searched through the crowd of Ashina swords for Mikazuki a second time. He even glanced about Izuminokami and Nagasone. The tachi wasn't among them.
"What're you doing, Mutsunokami?" Kashuu asked, arms crossed.
"Where's Mikazuki?"
Kashuu paused. That tachi's cryptic references to protecting things and the moon were obvious. The Sanjou swords checked their rooms and found no trace of him either; he had disappeared.
"Tsk." Ookanehira tapped his foot on the ground. "Why should we care where that Tenka Goken went?"
The Konnosuke, one white and orange, the other black, paced on the engawa. "The sages are ready to speak."
Two maids parted the shoji as the sages stepped out. Rinko wore a black kimono and scarlet obi; this world was so glaringly bare and white, she wanted her darkness to absorb this light.
Sen remained in her usual white robe and red hakama. Mutsunokami could only dream of her riding about in armour.
Rinko looked down at her Konnosuke. "Show the map."
The black fox's eyes glowed blue and projected a hologram before the swords. A citadel icon marked their homes at the centre while it was surrounded by three blue rings.
"The circles are the revisionists' army, divided up," said Rinko. "There are millions of them in each."
"They can be fended off if our teams spread out to fight them," Sen said.
"And confronted throughout the rings," Rinko said. They'd catch the enemies off guard, and they'd win. They were stocked with months of rations. The revisionists would pay for doing this to them. "Mikazuki Munechika has disappeared; if anyone knows what happened to him, let us know."
"Why should we fight and be on the lookout for that old man?" Ookanehira muttered. One of those missions was much more important than the other.
Sen pulled folded sheets out from her sleeve. All the names of their swords and teams were sorted. Once they were called out, they'd have to get equipped and leave for battle.
Rinko and Sen made their way down a cobblestone road towards a torii gate. The trees along the path were tall, thick, and covered in moss. They had grown green and large without a sun. The moon, on the horizon, stretched wider than the torii gate and trees, casting shadows on the foliage. Its light beamed on the saniwa, illuminating them.
The largest, strongest enemy was slain. The feed from the konnosuke showed it all. The revisionist's newest creation was a large, hulking warrior. His very existence was too heavy to bear, as his every step shook the ground. He oozed black smoke; he wouldn't survive for long, no matter what.
Mikazuki said something, did something as the enemy was about to kill the team out there. Kashuu and Mutsunokami were engulfed in blue fire. They withdrew their swords by an inch and their red and amber eyes blazed. They ran, leapt, and struck the enemy. The beast's armour, his form, split apart wherever they slashed.
The Konnosuke finally established a connection with the Time Government and confirmed the revisionist army's retreat. The citadels would be transported back to their world in a few hours.
No foxes or devices spied on them, so Rinko spoke.
"The Time Government was unhelpful." They could've figured out the enemies' plans on their own, so long as they scouted.
"We have no choice but to follow them," Sen said. They understood that since they were girls apprenticing under the eyes of the Government. No matter how hard the sages and swords worked, they were expected to do more with less while under evaluation. In the next meeting with the officials, so long as Rinko supported her, Sen would request a higher budget and renovations; they needed more rooms, stalls, and ration storage if this invasion happened again. The revisionists had billions more soldiers to expend for their plans.
"Perhaps they will reward us for our hard work. I would appreciate a tsurugi in my citadel," said Rinko.
Kashuu, holding the blade Mikazuki in its scabbard, reached the top of the stairs and stood under the pale torii gate. The moon cast his shadow forward. He straightened at the sight of the sages. As tired as he was, he had to present himself with dignity.
"All this trouble over a Tenka Goken." Ookanehira scowled as he stood by Kashuu. He patted dirt off his black sleeve.
"This could happen again." Kashuu turned to the tachi.
"Then don't give that old geezer a body." Ookanehira pointed at Mikazuki. "Bury him in a basement."
"That's not what I meant. Another Tenka Goken may run away to face the armies alone."
Ookanehira narrowed his grey eyes at Kashuu. Juzumaru would never do something so brazen. Hell, Oodenta knew better than to leave his storage shed without reason.
Hizen and Mutsunokami carried a limping Hakusan by his arms up to the gate.
"Master!" Mutsunokami sprinted to the saniwa. Kashuu walked. The moon remained large and cast its light behind the first swords of the citadels as they approached the sages. Despite everything that happened to them, Mutsunokami smiled and his amber eyes were on Sen as he stood in front of her. They couldn't hug or kiss, not with witnesses here. They'd speak later, love later.
"We all need to return to the citadels; the Time Government informed us we will be sent back to our world soon." Rinko's pointy black fingernail poked a crescent moon on Mikazuki's scabbard. "I will deal with this old man tomorrow."
"Ha ha ha." The sword in Kashuu's hands vibrated. "Let us celebrate the new moon, only for tonight."
Shichiseiken was added to the Ashina Citadel after the Great Invasion.
I wrote this before Juzumaru's kiwame was released. I expected the Great Invasion to happen again, but it didn't. Sorry for doubting you, Ookanehira.