Like everything else in your life, it went wrong pretty much straight out of the gate. And it was probably your fault.
Papyrus might have been too kind to be let into the Royal Guard properly, but he was excellent at sentry duty; his dedication was unquestionable, and he never got bored or left his posting without reason. The cold in Snowdin had no effect on him, and he loved playing with all the dogs. He was one of the few that could train properly with them, too; they had little to no self-control with their strength, but Papyrus was tough as nails. It was cute, in a sad, pretendy way; they thought they were playing, and he thought he was training.
So of course, it was Papyrus that actually encountered the human before you did.
Oh, you'dheard first, of course. Well, maybe not first. But you were definitely the first to actually see them come out of the ruins - your camera placement was exacting. You were halfway through a text to Sans when he sent you one. Most of his texts were only a few words long and had no punctuation or emoji, so maybe he could be forgiven for interrupting your newsbomb.
The rumour that a human was around spread like wildfire. There wasn't a monster in the Hotlands who didn't know about it inside of ten minutes. Did you hear? There's a human in Snowdin.
The skeletons are talking to them!
The spiders say they killed a froggit.
They killed Lesser Dog.
They're killing... everyone.
And all you could think was no no no this wasn't how it was supposed to be. But Papyrus was still talking to them, wasn't he? He was… buying them time, perhaps. Or maybe he was just being Papyrus. Maybe it was a little of both. You sent Mettaton out, just in case the puzzles helped, but he came back far too soon. The human hadn't looked left or right; had walked straight through every single puzzle in shoes covered with dust. There wasn't much else to slow them down, after that; Snowdin's residents were already fleeing.
You and Mettaton came to the same conclusion in rapid order; those people were going to need somewhere to stay. You weren't getting any more texts from Sans. There was a creeping, crawling feeling in your gut, of something horrible starting, so when Mettaton suggested you start preparing the Hotlands just in case, you agreed. There was plenty of room, after all, and the paths above the CORE were mazelike with dead ends and dangerous areas; you could easily section them off to provide shelter. You could keep people in or out if you really, really had to.
Even then, the full weight of your burden hadn't sunk in yet. Mettaton was immediately off being himself, brash and bold and making grand declarations of hospitality on the MTT station to anyone who might be listening. You were folding blankets and packing up dog food to send down to the basement, and wondering how Papyrus was doing.
So you were still in your labs, busy, worried and restless and filled with a crawling sense of dread, when you got the call from Undyne. Her words were a sudden nonstop stream that took you both several tries to make sense of. No, she wasn't coming by; she didn't have time to wait for the ferry, and in any case she wasn't going to join the evacuation. She'd rallied what remained of the Royal Guard and was sending them back to Hotland.
I'm going to stop the human, Undyne had told you. I told every monster in Waterfall to head there. You've got room, right?
Of course you did, but you couldn't anticipate thismany people. Your mind was reeling so much you barely noticed when Undyne continued speaking without waiting for the affirmative. All of Waterfall? That was dozens - hundreds? - more than those already in the Hotlands. You'd need more than a single floor for this, that was for sure. You'd need to close off some of the deactivated puzzle areas and put people there, too. You'd need to tell Mettaton to keep broadcasting for awhile. But that meant -
Me? Don't be ridiculous. This is just to get everybody out of the way. I don't want anyone else getting hurt. I'll take the human's SOUL, and bring it -
Something in your chest seized up at that. Don't lie to me, you told her, managing to stammer the words out even though it felt like your heart was starting up a race with your lungs. (It is the only time you have ever demanded anything, of anyone.)
Undyne had fallen quiet then, stayed so for so many seconds that you were worried she was going to hang up on you, or that something had happened already, and you didn't hear it, somehow, had missed whatever had happened and you wouldn't hear her. It nearly gave you a heart attack when she spoke up again.
She told you to evacuate everyone if this all went wrong. That she needed you to watch, just in case. That she needed you to watch because she had to know that everyone else would be evacuated, that you knew what was happening, that your eyes were on her. And she might have said more, but there was no time; she barely had time to make the call, and she knew you were one of the only ones that could find a way to get word to every monster in the Underground.
She had to go now, and get an update herself; Papyrus had missed his meeting.
She might have said more. You might have said more, but there was no more time.
Everything after that was a blur, remained a blur for years to come. The details were there but you couldn't bring yourself to put them back in focus, to make them sharp again. You see them the same way you remember your nightmares in the morning, vaguely, with terror, but without wanting to think so hard, to recognize why your sweat is cold and your heart is pounding.
The massacre had stopped eventually, inexplicably, in the Hotlands. It was too late by then to change much about the plan. Mettaton had already come to your labs - can I borrow a few things, Alphys, darling? - and he hadn't said what or why, but he'd disappeared, and once the human was in Hotland there wasn't many people left to evacuate in any case. Your success was mixed; you couldn't get Muffet to leave, and Gerson refused to leave his shop, but you'd finally gotten Bratty and Catty to leave their junk behind, and Napstablook had shown up with a dozen snails in tow, being pulled in a cart by a horrified but resigned Woshua. The last few had trickled in for the night.
Undyne never came back. Mettaton never came back.
You didn't know what else to do, but you managed to get everyone sorted out somehow, with blankets and food and a place to sleep. There are electric barriers surrounding every floor and guards behind them, hiding out of sight so as not to provide temptation. There are hundreds here - children, adults, shopkeepers, normal monsters; slimes on the floor and oddly-shaped birds perching on the rocks. They keep asking you questions and you don't know what to say, but you tell them they're safe, because what else is there, right now? You get some of the shopkeepers to set up rationing and help manage supplies, but that's all you can do.
Eventually Sans texts you again, telling you to reroute as many of the elevators as you can. You're so glad to see a notification on your phone that you get tears all over the screen.
You try not to let anybody see. There's enough to deal with; they don't need your problems. You bunker down with everyone else, and then there's nothing left to do but wait.
Later, after the longest night of your life, you finally get another message from Sans, saying he's on his way back, saying that the human passed through the barrier, the human is gone. And you know what that means, because you know all about the barrier, you know there's only one Boss Monster SOUL left, and he isn't in here with you.
You hug Sans when you finally see him. He lets it happen, he doesn't argue. You cry into the softness of his hoodie, and after a few moments, you feel bony fingers wrapping around your sides as he hugs you back.
I should have been there - is what you think. You don't have the heart to ask Sans if he thinks that, too, of you, of himself.
And later still, much later, there isn't much of anyone left, when the dust is finally washed away and you can bring down the electrical barriers and everyone can breathe again. You ask Sans what to do next, but he won't look you in the eye; he shrugs and gives you that never-ending skeletal grin and reminds you that people still need to be taken care of. And that he's never really been good at that, but everyone seems to like you well enough.
He's right, even after - what you did. There had been no hiding the experiments when everyone evacuated to Hotland. You couldn't leave them in the basement labs; you didn't have the heart to just abandon them, not when your hubris is what led to this in the first place, not when you have already created so much suffering. But no one questioned it then, when they had seen long-dead family members returned to their arms in so many pieces; and even afterwards, when resentment could have, should have settled in, it never did. The monsters are just… thankful. There are a few more living souls in the Underground.
Sans was right, really. Everyone could go back to their homes, but the houses are cold and dusty, smashed and robbed clean sometimes, and the Royal Guard is gone, all gone, with no one left to lead even if there was a leader. Even if she was here.
She isn't here; she'll never be here again. It is a realization that fills you and never goes away, after that horrible night. Grief overwhelms you at odd times ever afterwards, turning your words to dust, squeezing the air from your lungs when you least expect it; you could throw yourself from the edge of the waterfalls, dive into the lava of the Hotlands and take a swim - you can't help but wonder if the last Royal Scientist went through something like this, and that's why they disappeared.
Surely there is no good way to live through this pain, except that now people are looking to you for help, and you can't fail them. You can't, you can't, you can't; the words pulse in your soul with every beat of your heart. You're not Asgore, you're not Undyne, you're not even Mettaton, to provide the right words to everyone, to lead with strength and purpose, to give hope - but you will have to be enough.
They really do have no one else but you now.