This is the last chapter I have already written, so now I'm gonna have to start writing again instead of binge-reading fanfiction and manga
Toriel both loved and hated it when children fell into the Underground.
On the one hand, she loved children, and she was terribly lonely in her self-exile to the Ruins of the old Home. She loved the delight in their eyes as she baked a pie just for them or the peaceful way they lied down to sleep in her spare room.
A few years ago, there had been a child that had stayed down here for almost half a year before inevitably leaving. She had been a kusa genin, left orphaned by the war raging at the time. She had been a bright little girl, always carrying around that notebook of her's and writing notes on all of the strange plants and monsters that inhabited the ruins. Toriel had once tried to go through it while she was asleep, but she had found the writing completely illegible. Later, she had asked the girl about it, and she said that it was in code. When asked why, she said that it was so that other villages couldn't steal the secrets before she brought them back to Kusa.
The girl had then gone on a rant on how if her village could use the secrets of the underground, maybe it could become more than a battlefield for the larger hidden villages and maybe there would be no more orphans like her.
Toriel had smiled both warmly and sadly at the girl. It pained her to know that the human world was so harsh, even on innocent children like that Kusa girl.
About a week after she left, the skeleton from Snowdin that she would talk to through the Ruin's door had reported that Asgore had claimed yet another child's SOUL for his plan to escape the Underground.
That had been the fifth SOUL so far, though there had been plenty more children that had passed without their SOUL being taken.
The sixth SOUL had been from a nice little civilian boy that had run away from his neglectful father. He had brought along an apron and a frying pan, both gifts from his long dead mother. When he left after scarcely a week, it had only taken a day for her to hear news of a human SOUL taken by the royal scientist's robot in Hotland.
Toriel was determined not to let another child escape her grasp only to be used to wage a pointless, doomed way against humanity. She had steeled her resolve, determined to keep whatever innocent little child from leaving, even if it meant destroying the exit entirely.
She had not been prepared for the child to want to leave so soon. Toriel hadn't even expected the child to speak up at all after their complete silence so far.
"W-why leave so soon my child?" she stuttered, not quite knowing how to answer without seeming suspicious. "You look tired, why don't you at least stay the night? The pie shall be cooled in the morning and then we can eat it together."
He (it must be a boy with such a deep voice) raised the eyebrow over his good eye, looking straight at her and then glancing around at the kitchen, seemingly debating if staying was at all a good idea.
After a more than awkward wait, the boy answered with a shrug. "Do you have a spare room?"
"Of course my child; right this way," Toriel replied, taking the boy's hand, much to his annoyance, and guiding him to her well used guest room.
It was a homely little room, and Toriel was proud of herself for fixing it up after she moved here.
The walls were painted red, with a large matching carpet in the middle along with the thick red quilt on the twin sized bed sitting in the corner. In front of the bed was a small toy chest filled with left behind toys of the room's previous occupants. The room was lit by a tall lamp located across the room from the bed and next to the box of worn shoes of various sizes and styles that Toriel kept just in case a child needed a pair. All in all, it was quite the cozy little place.
The boy, Toriel resolved to learn his name sometime tomorrow, was closely observing the room. He gave an empty frame sitting on a desk a curious look, but he seemed mostly unimpressed.
"I'll leave you to get settled. Goodnight my child," Toriel spoke softly as she left the boy be. She needed to talk to someone.
Sans could always feel it when a SOUL passed through the barrier. There was this slight shift of magic coming from the barrier, almost a ripple of sorts that changed the air ever so slightly.
The change would be even stronger when a shinobi fell through.
Sans had been a much younger skeleton the last time there was a ripple this intense.
Like always when humans fell down, Sans took the first excuse he could find to go to his knock-knock joke door. If Papyrus came to his station while his meticulous patrol as a royal sentry, he would find a note saying that someone had replaced all of the snow in the forest with flower and Sans was definitely not the perpetrator and he was certainly just putting all of the snow back and not just goofing off.
Sans took great pride in his ability to come up with nonsensical excuses.
He reached the door quickly with one of his infamous shortcuts and knocked lightly on the thick set of purple doors. They were a familiar sight, reaching more than twice his height (though that wasn't saying much considering how short the skeleton was) and framed by two large pillars with a delta rune at the top. He had visited this place many times throughout the years, and he looked forward to his talks with the lonely woman on the other side almost as much as he loved hanging out with his brother.
There was no answer, which was disappointing, but not unexpected. Whenever a human falls down, the lady is always busy taking care of the child, and she always tucks them in bed before she goes off to have their daily joke session.
Whenever that happens, Sans takes the time to lean up against the door and grap the extra nap time.
And nap Sans did, snoring loudly despite his lack of nostrils, for a good few hours before he was jolted awake by two knocks on the door and a soft, familiar voice saying, "Knock knock."
"Who's there," Sans replied, spreading his permanent cheshire grin.
"Dismay be a bad joke, but I think it's funny."
Sans let out a low chuckle that was soon accompanied by the undignified snorts of the lady behind the door.
"Knock knock," Sans started again.
"Just passion through and thought i"d swing by."
They both went back and forth like this for a while before the lady suddenly sombered.
"A human just fell down today," she announced solemnly.
"I could feel their SOUL passing through."
"I haven't felt the barrier ripple like that in a long time."
"He…" she started, her speech trailing off, "he reminds me of Chara. He's someone that grew up too fast. Someone the world threw away without a care, and I can tell he wouldn't hesitate to dust anyone in his way. I can see the weight of the world in his eye."
'Eye?' Sans thought. He had seen worse than a lost eye, but it was hard to imagine such an injury on a child, shinobi or not. Hardened warriors lost eyes and limbs; Sans worried if the kid was safe to be around, especially after what the lady said about the kid.
"Can you promise me something?"
"Promise me that you won't let the boy perish like the rest. I don't want another life to end too early."
Sans hesitated. Unlike the lady, he had a particular distaste for humans. They had strong SOULs, capable of more heinous crimes than a monster could even think of committing. There had been very few monster deaths thus far, but that didn't change the fact that they were capable. Sans had not killed any himself out of respect for the ladies wishes, but protecting a human child seemed beyond him.
Then again, he cared more for the lady than his grudge against humanity.
I'm going to clarify that I have a headcannon where either only children can pass into the barrier or the barrier somehow attracts children, probably both. That's why monsters have come to expect children falling down instead of adults.
Monsters who can feel it when a SOUL falls into the Underground are just monsters with a high natural sensory ability. There are more that can feel it besides Sans and Flowey, but not all monsters who can sense SOULs passing through know what they are feeling, so they write it off as the monster equivalent of a gut-feeling that something is going to happen.
The barrier doesn't react to inanimate objects with no SOUL, so luckily, nobody feels it when garbage falls into the dump. Speaking of the dump, since this is in the Naruto universe, I imagine that a fair amount of weapons and sealing scrolls have fallen down there, and I imagine that there is an entire profession dedicated to studying human magic (chakra) that visits the dump often for research.
Also noted: the mad dummy will probably have more than one knife because they are all over the dump
Also also noted: Undyne has probably found at least one giant anime sword. She will most likely be secretly excited to see a human with a sword instead of just those puny knifes.