At the very end of the underground where Bill sans has hold everyone into a corner as he was going to successfully obtain all the human souls to cause chaos on the surface until a voice shouted on the top of their lounges to get his attention

Frisk: wiat!!! Let's.... Make a deal... 1 that will prove that you don't need those human souls!
Chara: Frisk!! Are you crazy?! You can't make a deal with him?!
B!Sans: ohhhh sounds like someone's bold huh? And what could you possibly offer that's more powerful than human souls?
Frisk: I hold the knowledge of how you could destroy that barrier without the human souls! With that power you don't need the souls at all! All I ask for is... That you leave the underground... And never attack us again...
Asriel: Frisk don't!
B!Sans: interesting... Hehehehe... Alright kid, you have a deal
Chara: no! Don't!!

Bill sans fly down and shook Frisk hand, for a moment there was silent for a few seconds until Frisk pulls Bill sans down with a huge grin on their face

Frisk: thanks for the powers old pal.

A huge explosion happend and everything went blank

B. Frisk: and that's how I ended up taking his powers and make them better! Hahahahaha! He dosent know that ofcourse~ I had to delete all their memories of such event~
???: I see... So what did you do after gaining his powers...?
B. Frisk: simple, I gain the knowledge of alternative universes! So ofcourse I was curious and travel around to see it all for myself~ and geuss which au peeked my interest...
???: that's when you saw me huh...
B. Frisk: mhm~ you and the poor thing were dying on the burning ground, ofcourse I had to help~ in the end I could only do so munch for the little ones injuries, this power isn't suited for healing you know~
???: I understand...
B. Frisk: than again even after we had a goddess of life on our side that child still couldn't heal his wounds... It seems interesting no?
???: it is to be expected... Nightmares powers were out of control after he bite that apple... Naturally causing him to create such injuries...
B. Frisk: mhm, so boss~ what do you weigher our next move to be~? We have all the Frisk from any universe within our grasp, they will do anything under your command sience you are their savior~
???: for now... We should stay hidden... We can't risk letting those Sanses know that we exist... It's too risky...
B. Frisk: oh relax, this little dimension I made for all of us is safe for the kid, as long as he remains here, no harm will come to him
???: I know... That's why I'm willing to do anything to fix all of this... I will end his pain and suffering... I will end all of our pain and suffering... I just need time...
B. Frisk: oh don't worry... You have all the time you need in the multiverse... Nightcore

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Nightcore: no no no no no...  Daydream please... Stay with me... I'm sorry... I'm sorry...! Why... Why is he doing this?! Please... Stop..! Nightmare!!!
B. Frisk: stay still you 2, this will shook you~

Nightcore: huh?! W-who... Who are you?! Where are we?! Why d-do you...
B. Frisk: look like you? Let's just say... We're the same person my dear Frisk
Nightcore: wha... Uh- Daydream?! M-my brother! Please you have to help him!!
B. Frisk: here let me see him

B. Frisk: this isn't good...
Nightcore: what's wrong? What's going on to him?!
B. Frisk: ah his injuries are deep with corruption, I manage to heal some of it and stop the corruption from flowing any deeper but that's all I could do... I'm afraid he's in a deep sleep
Nightcore: no... There has to be a way right?! We can save him right?!
B. Frisk: there's very little we can do, I'm afraid there's nothing else I can do to help
Nightcore: no... Why...? Why is this happening to me?! I didn't want this...
B. frisk: none of us did my friend... You see... Your not the only Frisk who is suffering right now...
Nightcore: I'm not...?
B. Frisk: I'm afraid not... Many other timelines and universes of us are suffering from their own actions, losing friends and family by their own hand... It's all quite tragic... The choices weren't even in their hands...
Nightcore: why... Why must we suffer like this...? It's not even our choice...?
B. Frisk: there's a simple explanation to all our suffering honestly... It's because... We are nothing more than characters in a game~
Nightcore: w-what?!
B. Frisk: yes we are nothing but a puppet character to be controlled by someone else just to go around causing misery because we're just... A game... Tragic is it not? And it all started with this button
Nightcore: a... Reset...?
B. Frisk: this is a power that we hold in every multiverse wield, it hold the power to restart the game... We can chose to save everyone... Or kill them all
Nightcore: what...?! I don't want to kill anyone!
B. Frisk: you don't have to, now that your world has been destroyed by its own Sans, there's no one here for you to harm, you are the soul survivor

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B. Frisk: hehehehe you were so poor and innocent... And so determined to save everyone~ oh how I wish I had such wishful thoughts
Nightcore: *sigh* this is what I must do, I must help them all build a better world for all of them... They cannot stay suffering in their own world like this forever... Our goal is too dangerous so that's why I have a task for you Risk... I do hope you will take it seriously
B. Frisk: hehehehe Risk huh...? I like it, I'll do my best to follow my leaders orders~! You know I do
Nightcore: don't worry Daydream... I'll save you...