Credits to @MysticalStoryWeaver for helping me out with this fight scene. Excerpts of the fight was taken from her oneshot Strings of Fate.

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     Sarah dusted her pants off as she took a look at her surroundings. Asking Nemuri to teleport them to a random part of the multiverse was, quite frankly, a big mistake. Unlike the ancient demon, Sarah wasn't familiar with how things in the multiverse worked. Hell, she wasn't even introduced to the concept until a month ago.

Was teleporting to that one place when they were younger count as being in the multiverse? The place with all the weird glyphs and that Belos guy who wanted her dead?

"Yes, that is part of the multiverse," Nemuri said inside her head, "I teleported us there after I finished off Amagakure."

Hm..that made a lot of sense.

"But why exactly are we in the middle of the fucking woods at night?" Sarah asked them, "This doesn't make any sense!"

"Well you asked me to teleport us somewheres. You should've been more specific as to 'where'," Nemuri responded in an annoyed tone, "Sheesh."

Sarah rolled her eyes, "Might as well explore."

"Be careful," Nemuri warned, "I sense a powerful presence nearby. Nothing that we can't handle though."

Sarah took out their pair of black gloves and donned them. It's best to be prepared for any upcoming attack.

Continuing her walk, Sarah could hear some faint shouting in the distance. A battle, perhaps? But who could be fighting in the middle of the night?

Rushing over, Sarah quickly came upon a startling scene. A boy with crimson hair engaging in combat with a small child that looked eerily similar to a spider. Looking up, Sarah realized that there was a girl being held captive – bound by spider threads, her blood slowly dripping to the ground.

Pulling out a singular kunai from her pouch, Sarah climbed up the tree and cut the girl from the threads. The girl fell to the ground below, a thud being heard all around. The noise caught the attention of both boys, the white-haired one making an angered expression on his face.

Staring at the girl, questions began flooding in the boy's head. Who was she? Why did they smell like a demon even though she wasn't?

'WRONG WRONG WRONG! NOT OF THIS WORLD! SHE IS NOT OF THIS WORLD! IMPOSTER! MUST BE ELIMINATED!'

Tanjiro looked at her with a puzzled expression on his face. They definitely did smell like a demon. She probably is a demon, but something about her scent didn't add up. It wasn't like any of the other demons he's fought before. That was weird.

Hopping down from the tree, Sarah was quickly met with three sharp threads coming at her. Sarah sliced through them with ease, getting into a battle stance as they eyed the boy down menacingly. Of course she wasn't going to get away without engaging in combat. Typical.

"Why did you free her? She is my sister," the boy spoke.

"Sister? You two don't look related," Sarah said.

"Shut up! You don't know anything!" he snarled, "I want their bond! I want what they have! No one in my family, they would never do what she did for that human there! And she's a deam-"

"Being a demon doesn't have anything to do with this," Sarah snapped, "She is her own person and you're treating her like she's some sort of object! I may not know much about healthy family dynamics but this sure as hell isn't one!"

"Shut up! Blood Demon Art: Web Toss!" the boy created multiple sharp webs and threw them at her.

Sarah pulled out some sort of seal from her kunai pouch and attached it to her kunai, throwing it at him. Vines sprouted up from the seal, wrapping around the boy. Using this opportunity to check up on his sister, Tanjiro headed over and helped Nezuko up. Thankfully she's safe from Rui's grasp, but why did that girl go out of their way to help them?

Roaring like a beast, Rui used Blood Demon Art: Cutting Thread Rotation to cut the vines apart. Before he could react, Sarah quickly appeared right in front of him with her fist glowing in some sort of mysterious green aura.

"Shannaro!" Sarah yelled as her fist made contact with Rui's body, sending him flying through multiple trees.

Rui quickly got up, trying to gather his bearings. Before he could, Sarah appeared in front of him again and roundhouse kicked him to the ground.

'She's too strong! I can't take her on by myself,' Rui thought as Sarah quickly placed a stasis seal on his back.

"Well, that was a good warm up," Sarah said as they dusted off their clothes.

'Warm up? That was a warm up to her!' Rui gritted his teeth in anger.

"You're quite weak," Sarah said as she glared down at him, "To put it bluntly; you're holding yourself back."

"I am not weak!" Rui shouted, "I am a member of the Twelve Kizuki! I am stronger than most of the demons here!"

Sarah rolled her eyes, "Lower Rank Five? Seems pretty weak to me. I could probably solo your fellow Kizuki's without breaking a sweat."

"Shut up!" he roared, "You think you're so strong? My Master is the strongest being out there! He could defeat you in one hit!"

"That is if he manages to land a hit on me," Sarah picked the boy up and threw him over her shoulder, "But he isn't here, is he?"

Rui remained silent. There wasn't much he could say. She was right; he wasn't here.

"I'm sure he can make do with one less demon," Sarah said as she opened up a portal.

"What do you mean by that?" he asked.

"Well, you're most likely going to die if you stay here. I'm going to take you in under my care and keep you," Sarah grinned as they stepped through the portal, closing it behind her.

"Wait, what!" Rui shouted as Sarah sat him down, peeling off the stasis seal.

"Welcome to your new home, spider-kid!" Sarah said as Rui sat there mortified, "I hope you enjoy your stay."

Rui highly doubts he will.