Chapter 1: Good Times and Bad Times.
"it's a beautiful day outside. birds are singing, flowers are blooming. on days like these, kids like you..." you know the drill.
"Your threats no longer frighten me Sans. You already know that I already know exactly what transpires in this hallway."
It was a battle that had happened thousands of times before. Sans the skeleton facing Chara, the demon that comes when people call its name.
"really? then tell me kid, what happens?"
"You fight me. You give me a 'bad time.'"
"huh. funny you should use the word time."
"What?"
"ya see, i got this friend. they're basically the controller of all of time. one could even say they are time itself. now, this being is basically immortal, so i figured, why put myself at risk, when i can just send time to kill you."
"You're joking," said Chara, nervously.
"well, ya know what they say." Sans winked. "time will tell."
At that moment, with a flash of yellow and cyan light from his left eye, Sans disappeared. In his absence, Chara felt a roar so loud it shook the hall.
"No!" yelled Chara. This wasn't how it was supposed to go, it wasn't how it was supposed to be.
* Dialga used Roar of Time.
A blast came from the mouth strange creature who had appeared before Chara. It hit Chara directly, knocking them to the floor. With the fallen human's last moments of consciousness, they tried desperately to RESET things back to the way they were.
* But it didn't work.
(((So, while I have a logical break here, I believe that I should put an Author's Note here. This is Timeline Jumping Left And Right [TJLAR]. In order to understand this story, you will have to understand many things about both the Undertale and Pokémon universes. You will also have to understand that I sometimes not use the é in Pokémon. I may also unintentionally have content that goes against the canon of one of the universes. However, I do not doubt that these faults will be forgiven. Will contain spoilers.)))
A different timeline...
In another timeline, which from a specific point of view was 2 RESETS before the one described earlier, The human known as Chara spent 2 days in the ruins as opposed to the usual relatively short period of time. During this time, Sans became very busy.
When he went into his lab one day, he saw something different then what he usually saw. He checked his readings on the timeline, and was surprised. Rather than seeing that the entire universe would be destroyed that day, he saw that it would be destroyed two days from then. This gave him hope. Only a tiny spark of hope, but hope nonetheless. He immediately got to work on an experimental machine he had worked on for years.
He called it the Universal Timeline Relocator. It was a very dangerous machine that could, if used improperly, erase someone from having ever existed. If it worked properly, however, it could take an object, and transport it to another universe. However, it did not transport the matter into the other universe, it transported the idea of the object into the universe so that it would end up occuring in that universe naturally.
Through the hours Sans spent working on the machine, however, he worried. If he failed, not only him, but the entire mountain would have never existed. Would Sans end up like Gaster, who ended up spending his entire existence in agony. At least Gaster existed in some form, still had a past. If Sans failed, he would be condemning an entire species to oblivion.
Still, Sans worked through the three days. Even when the human exited the ruins, he kept working. They walked through the entire Underground, killing everything in sight. Only when Chara finally reached Judgement Hall, did he finally do what he finally decided that, functional or not, he would have to activate the machine.
Present Time
Sans took the SOUL of the human to the Throne Room. There he saw Asgore, speaking with someone. The conversation was quiet enough that he couldn't make out what they were saying. However, at the end of the conversation, he heard Asgore say "Curious, I've never seen a plant… cry before."
He turned around. "Sans, I cannot tell you how relieved I am to see you." Upon hearing this, Flowey emerged from the ground. "Sans?" asked the flower, "is that you?"
"yep. It's me." A SOUL formed in his hands. "i took care of it for ya. well, i can't take all the credit. i did receive help from a legendary pokémon, but in the end, what matters is, we can leave."
"Ah." said Asgore, who looked both sad and relieved at the same time. He placed the SOUL in a container. "May I ask, where the body is?"
Sans's eyes went black. "Don't worry about it." Asgore immediately dropped the issue. However, still Asgore hesitated. When he felt he should be experiencing relief, there was only more fear and regret.
"my guess is..." said Sans after a few minutes of silence, "you don't wanna use those souls." Asgore nodded. "well then, why don't i do it?" Asgore looked at Sans, his eyes pleading with Sans to do it.
They went to the barrier, where they put the seven SOUL contain in a row. With a wave of his hand, all of them opened. They circled around his hand until they touched it.
When they did, the sheer power of 7 human SOULs rushed through his bones. His jacket transformed into a much longer coat, with decorative silver lines appearing. His slippers became purple shoes, and his pupils changed color, the left one becoming yellow and his right one becoming light blue. Symmetrical black lines appeared on his skull, branching out from his eyesockets in the direction opposite from the other eyesocket.
"well then," said Sans, in a voice that echoed throughout the room. "guess i should, break the barrier and resurrect everyone who died this past day then?"
"That would… probably be the best option," said Asgore.
Sans sent a burst of power into the barrier. A flash of light reflected off of the barrier. When it faded, Sans's pupils had returned to normal color (though the other changes remained), and there were now, rather than two monsters and a flower, 4 monsters in the room. Asriel, Toriel, Asgore, and Sans.
"well, have a nice life. i gave you a new universe, where humans will hopefully be more tolerant, and here's your son back. as for you and tori's relationship," he said, speaking to Asgore, "eh, not much i can do."
While Sans was speaking, Toriel muttered to herself in confusion.
"now if you'll excuse me. i need a podium and a place to speak on live television."
"SAY NO MORE," said a robotic voice, "METTATON IS AT YOUR SERVICE." Mettaton, having announced his presence, entered the room where the barrier once existed.
"ah, you're a sight for eyes who just watched everyone get brutally murdered. come on, we have thirty minutes to prepare everyone. while you're doing that, i'll be going to alphys's lab."
After fifteen minutes the Dreemurrs went to the place where Alphys had evacuated everyone to see Sans speak. After fourteen more minutes, Mettaton went up to the Podium in front of the crowd. "WELCOME, CITIZENS OF THE UNDERGROUND. I AM HERE TO SAY THAT SANS THE SKELETON IS READY TO SPEAK."
Sans, still in his godlike form, only now wearing a tie, stood at the podium.
"heya," he began, "many of you once knew me as that one comedian from mtt resort," a few people laughed, "now you may know me as one of the people in charge of the evacuation, or by my title of royal judge. however, as you can see my physical appearance has changed a bit. this is because the seventh human, the one responsible for the tragedy that happened today, is dead." People cheered. "as you almost certainly know, this means that the barrier has been broken by a being of unimaginable power. this being would theoretically have the power to not only break the barrier, but also resurrect many monsters, including all of those who died today as well as the prince, asriel dreemurr." Many monsters were shocked to hear the name mentioned.
"through the absorption of the human souls, i did all of this. i also brought the queen back from her hiding place in the ruins. sorry about that," he said, looking to the royal family. Everyone else turned to see them as well. "anyway. to get to the main point, all of you are free. i can say with near absolute certainty that there will not be another war. so don't see this as the end of imprisonment, because then you gotta think about how humans imprisoned you. instead think of it as a beginning. an entrance into a new society, full of new humans. 'cause i guarantee, when you leave this cave..."
Sans winked and chuckled "you're barely gonna recognize 'em."