This story is heavily inspired by "The Great Decension" written by 2eLegereYoukai. You can find their work here on FFN or over on AO3.
"Listen to me, Aether." Lumine spoke, "I have already traveled through this world once. Once you reach the end of your journey as I did, you will see for yourself the true nature of this world."
The Abyss Herald behind her had begun conjuring another portal as she finished speaking, ready to leave. Aether, the Traveler, Honourary Knight of Monstadt and Saviour of Liyue, couldn't bring himself to move after his sister.
"So we'll meet again." She said, the portal behind her growing larger, "Though we need not rush, brother. I have more than enough time to wait for you."
At these words, with all the confusion and frustration brewing up inside of him, Aether ran to his sister. Why is she so invested in this world and her battle against 'Destiny'? Why join with the Abyss? How did Dain fit into all this? He needed answers now.
Lumine stepped towards the portal, "We have always had enough time." Aether hastened in his sprint, meanwhile Dainslef used his own powers to rush past him, catching the portal.
Aether as well seemed to be able to catch the portal before it closed off, but as he leapt into it, it became very clear that something went wrong.
2nd of May, 1916
11:43 CEST
Paris, France
Pierre Roques, War Minister of France, sat in his office in Paris. The Germans seemed intent on bleeding their forces at Verdun, and the timing couldn't have been any worse. They had been preparing for an offensive at the Somme, where French and British lines intersected so as to make one united push against the Invaders together. That blasted Falkenhayn's attack put that plan on hold. He and many others had been urging the British to go forth with the plan to attack at the Somme. Perhaps a British attack would relieve France's forces at Verdun and force the Germans to focus on both sectors. Every time however the British kept delaying, saying they needed time to prepare for it as they couldn't rely on the French forces as originally planned, and that they were waiting for a new weapon that wouldn't arrive until August.
Roques didn't know if they could hold out until August. Generals Pétain and Nivelle had taken over commands of Army Group Centre and the Second Army respectively. Roques could only hope and pray that they would be able to regroup and hold the line. Caught in his own thoughts, he was brought back to the real world when he heard harsh knocks against his door.
"Come in." He said. The door was roughly opened, revealing a young officer, "What is it?"
The Officer seemed out of breath, taking a few seconds to breathe, "S-Sir! There's a crisis on the West side of the city, near the Arc de Triomphe!"
"What kind of crisis?" Roques asked. Had the Germans resumed their bombardment? Has the Arc been struck?
The Officer seemed to hesitate, unsure of what to say. An urging from Roques brought his words out.
"M-Monsters! Monsters are attacking Paris, sir!"
Monsters?
"Monsters? What do you mean? How?"
"A portal- a portal has appeared within the Arc de Triomphe." The Officer said, "The… monsters are coming from it, sir." The Officer went on to explain the details of what was currently occurring. The portal had opened not even 10 minutes ago. The first to cross it and enter Paris was a blond boy and a small fairy, the both of whom seemed confused and lost. They apparently spoke English, yet didn't know what the language was called, nor anything else about the world. Police were in the middle of handling the blond and the fairy when the first of the monsters, a barbaric looking humanoid wearing a mask and wielding a club came through. From what the Officer described, there was already a small horde of these monsters within Paris, and it was growing.
"What are your orders, sir?" the Officer asked, having finished explaining the situation.
"Have whatever Garrisons we have deployed to the Arc. Have the Police and Fire Department evacuate as many civilians as you can away from the area."
"Yes, sir!"
2nd of May, 1916
11:31 CEST
Paris, France
Lumine wasn't here.
That was the first thing Aether realised as he crossed the portal. He didn't know exactly what he'd see across the portal, but he had hoped that he'd be able to catch his sister. Instead he found himself under a tall arch, surrounded by a street and tall buildings. The locals seemed just as surprised as he, pointing to him and Paimon, and to the massive swirling portal within the arch. A few uniformed men armed with what seemed to be small multi-barreled muskets approached him carefully, shouting at him in a language neither he nor Paimon could understand.
"Mains en l'air!" they'd say, "Mains en l'air!"
Aether and Paimon raised their hands in the air, hoping to diffuse the situation. Paimon tried to speak to them, saying "We're not going to hurt anyone!" and "Please put down your muskets!". One of the locals then approached one of the uniformed men and spoke to them, getting a nod before they turned towards Paimon.
"Uh, hello!" they began, "I am Raimund, what is your names?"
Paimon almost cheered at finally being able to understand someone, "This is the Traveler, Aether," she gestured to him, "and Paimon is Paimon!"
"Yes, Aether and Paimon," the man spoke to one of the other men in their own language, Aether noticing some similar words that he could make out, before he turned back to them, "These Officers here would like to know how you got to Paris."
Paris? That must be the place they found themselves in. Paimon tried explaining how they were trying to find Aether's sister, and that they didn't know how they got here, but then Aether recognised a sound from behind him. Materialising his sword, Aether swiftly turned around back towards the portal, cutting down a Hilichurl as it crossed.
Then another Hilichurl came through, which Aether defeated, and then another, then another. Then the Officers with their muskets began firing. More and more Hilichurls crossed the portal, pushing Aether and the Officers back. One of the Hilichurls tried to attack a nearby local who was trying to run away when Aether, using the power of Anemo, propelled himself forward, delivering a slash, killing the Hilichurl.
"Standez vous!" the Officers shouted to the civilians. Aether busied himself with fighting off a Mitachurl that had crossed the portal, creating a swirl of Pyro from a nearby Hilichurl with a flaming club to set its shield on fire.
"Windblade!" he called out, letting go of the swirl and letting its infused Pyro lash out towards anything near. Aether then called down a Geo meteor unto the Mitachurl, crushing it and the Hilichurls around it. Another Mitachurl, a few Samachurls, and several more Hilichurls crossed the portal shortly after. Aether called upon the power of Geo, punching the ground underneath him, causing ripples in the earth around him and Geo crystals to form, blocking some of the Hilichurls from moving forward.
The constructs wouldn't hold them forever, however. Aether readied his blade, dulled from countless other battles before, and used the power of Anemo to send a whirlwind towards the Hilichurls, picking up some Hilichurls and Samachurls and pushing them back across the portal, leaving the Mitachurls to deal with.
"Allez allez allez!"
All of a sudden, men in light blue uniforms and blue helmets, armed with longer muskets appeared. Dropping out from what looked to be trucks, these men formed firing lines, and everyone around hurriedly ran away. Shouting "Feu!" in their language before firing volley after volley into the Hilichurls. The shields wielded by the Mitachurls were able to be pierced by the blue men's repeating muskets, shredding them into splinters. Some brave Hilichurls tried charging towards their firing lines, only to be stopped in their tracks by the steady staccato of fire.
With many of the Hilichurls turning back towards the portal, the blue men began inching forward, steadily regaining ground while continuing to fire into the retreating horde. Aether had stood by once the blue men began firing, not wanting to be caught in the crossfire. The sight of the continuous fire they were able to put down range was something Aether very rarely ever saw in his travels across worlds. On every other occasion entire blocks of men would need to constantly rotate the front line to keep up the same volume of fire as the two ranks that these men had formed. Soon enough, the Hilichurls were all pushed back into the portal, leaving behind the aftermath of their attack.
Some Officers had been clubbed to near death, others not so lucky. One Officer had caught a Mitachurl axe and had died on the spot, another was tangled in dendro vines a Samachurl had conjured, his uniform torn and himself being covered in cuts and thorns.
Unfortunately, despite the best efforts from Aether and the Officers, a few civilians had been caught in the mayhem. While most were alive, the injuries they sustained were ghastly. Some had broken limbs and ribs, some had lost an eye, some had been paralysed from either the sheer shock or from being attacked physically.
As he was taking it all in, Aether heard someone call out to him. Turning around, he saw an older man in a more ornate uniform similar to the soldiers from earlier. The man spoke in his language to someone to his side, who Aether assumed to be his translator. Said translator then spoke to him.
"The Minister of War, Pierre Roques, would like to know why you have come here." the Translator asked.
Paimon, who had just now returned to Aether's side, spoke, "We don't know how we got here. That portal brought us here."
The Translator spoke to the older man, Roques, before then turning back to them, "Where does this portal lead?"
Paimon turned to Aether, "Should we tell them, Traveler?"
Aether thought for a second, thinking back to his sister and their conversation. Notably, the distinct lack of a massive Hilichurl horde. There was only one place he knew had a large number of Hilichurls.
"We don't exactly know where the portal leads." Aether told the translator, "but I might have an idea."
City of Mondstadt
Amber was running towards the Knights of Favonius Headquarters, having just returned to Monstadt after making a scouting run. All the Hilichurls from Dadaupa Gorge were caught up in a stir, and there was a massive swirling portal at the centre of it all. From what she could see, the Hilichurls were all going into the portal to whoever knows where. Amber thought for a second that the portal could be leading to Mondstadt, and hightailed it back, only breathing safely when she saw that the City of Freedom wasn't under a tidal wave of Hilichurls.
Nevertheless, she needed to tell Acting Grand Master Jean about the portal. For all she knew, another portal could open up wherever in Teyvat, causing havoc wherever it does. The Knights charged with guarding the Headquarters were all used to Amber's antics, and only stopped her to ensure she didn't just sprint straight into Jean's office before letting her into the building.
Amber delivered three knocks on Jean's office door, before hearing Jean call her in on the other side. She then walked in, giving the Knights of Favonius salute.
"Master Jean, I've come to report on my latest scouting run."
Jean, who had been busy signing off on some papers, nodded, "What do you have to tell me, Amber?"
Taking a moment to clear her throat, Amber spoke, "There's a large portal in the middle of the Hilichurl camps in Dadaupa Gorge."
Jean took a few seconds to process what Amber had told her, "A portal?"
"Yes. I saw Hilichurls go into it and vanish."
"Do you know where the portal may lead to?" Jean asked.
Amber shook her head, "Sorry, no. I was worried that it led to Mondstadt, that was why I came here in such a hurry."
"This is very concerning…" Jean said, "I'll dispatch Eula's squadron to survey Dadaupa Gorge. You and Kaeya will take an equal number of Knights to patrol the rest of Mondstadt in case that portal does lead somewhere here."
"Yes, ma'am!" Amber saluted. About to say something else, she hesitated, something Jean picked up on and urged her to continue, "What's the plan for if the portal does open up somewhere near?"
"Depending on its proximity to the City, we'll deal with it accordingly." Jean answered.
"…You don't know what to do, don't you?"
Jean let out a sigh, "We'll cross that bridge when we get there."