A few more years later (you are 9):
Once error had come back from destroying whatever au he was destroying you decided to pop the question that you had wanted to ask for years

"Papa error,can I go to underfell"

"*sigh* I knew you'd want to leave the anti-void some day,ya know what? Sure,let's go"

"Fuck yes!"

"What did I say about the swearing"

"Sorry papa"

Error made a portal to underfell and the two of you stepped through it,appearing at the golden flower patch clearing where the human falls into the underground

"Okay kid,I'm gonna leave you here,just go ahead and make your way through the underground like the human does but in any way you desire,if you need my help or are done with the run earlier than I expected then just tap your necklace twice and I'll be there"

"Got it Papa error,See ya later"

You wandered around for a bit after killing flowey who was in the room after where you started,once you had killed pretty much all of the monsters in the ruins you made your way to toriels home,you killed her on sight and opened the heavy ruin doors,it wasn't too hard for you since the decade of training had made you extremely strong and skilled,your magic had advanced since you had discovered it,now your claws and teeth can glow many different colours,red is super sharp and orange is hot like fire,yellow can be shot out of your hands like bullets and green makes plants grow,light blue is venomous and dark blue can make a shield,last but not least purple has an anti gravity effect on anything it touches which proved useful for disabling enemies when you're attacking them...

You stepped out into the freezing cold snow,walking forwards as it crunched beneath your feet,you then hear the sound of footsteps,but not your own,you knew that it was this universe's sans since you had watched the timeline play through so many time before,you ignore the sounds and continue walking until you get to a bridge

"Human,don't you know how to greet a new pal? Turn around and shake my hand"

You turn,catching him off guard since what he was looking at was not the human he had expected

"Wha- you're not the human? But how? The timeline-"

"Is messed up,I know,that's my bad,sorry dude,but your human isn't coming,i interfered and highjacked this run"

"Who are you?"

"Isn't it obvious? I'm your worst nightmare,now I'll just be on my way,see ya"

"Uhh see ya kid"

You walk away from the sans nicknamed 'red' and continued on,but instead of killing monsters you decided that you wanted to do something else,just killing them had gotten boring back in the ruins,you know that error had told you not to since one,you didn't know how to do it properly and two,you were too young to battle ink but you really wanted to try destroying this au,so you used your anti-gravity claws on all the monsters you encountered besides sans,after 'playing' with this universes papyrus,by that you mean you just went along with his puzzles,or shall we say 'death traps',in order to humour him,this earned you an ounce of respect from his brother

Once you had arrived in snowdin though he tried to 'capture you',just like in every other run,even though you weren't even human he still thought that a powerful young monster would be good as a pet,plus you were a cat monster so there was also that,you decided to humour him again and let him do so since you thought it might be fun,you could speed through the rest of the run anyways...

Back at the skelebros house:

"PET,WHERE ARE YOU"

"Calm down bossy pants"

"Pffft Wow"

"URGH SHUT UP SANS ITS NOT FUNNY"

"What do ya' want"

"JUST COME HERE"

underfell papyrus was sat on the couch ushering you over,you walked over and he grabbed you,picking you up and placing you down on his lap like a literal cat,which you were,and stroking your head,you purr and sans just stands there trying to hold back his laughter at the adorable scene that was playing out before him,heh,they really had no idea what you were planning...after petting you,underfell papyrus goes into the basement to do whatever shit he does,I honestly don't even want to know,I've heard screams from in there...and moans...*shudder* no thank you...I may be nine and a complete sadist but no,just no,I don't want to have that picture in my mind...It had been a few days and papyrus had grown to love you (platonically),he never wanted to admit it but he thought of you as a friend,sans,despite also having similar feelings,always kept up his suspicions towards you,clever bastard...

You decided that even though you weren't down there you still needed out of this damn house so you went out on a stroll through underfell,not the best idea but you could easily protect yourself so you couldn't really give a shit,you noticed that sans had gone to chillby's while papyrus was petting you so maybe you could join him,Sure,you were nine,but you had always wanted to try alcohol anyways so you rerouted yourself towards the flame monsters establishment...setting foot in the bar all eyes immediately shoot over to you who just flips them all the bird and saunters up to the seat next to sans,sitting down and catching his attention

"So (y/n),I have to ask,who the fuck even are you"

"In all honesty I don't really know,never have,who do you think I am sans"

"I really have no idea but I haven't heard shit from toriel and the kid seems nonexistent,where are they?"

"Dead obviously"

"You...killed them..."

"Well no shit Sherlock"

"Why"

"Why not"

"How did you even get here"

"Birthday present"

"What does that even mean"

He says,slamming his hand down on the table

"It means that it was my birthday and papa let me come here and wreak havoc"

"Who's this 'Papa' then"

"Well if I told you then you would probably just kill or maim me"

"Sorry kid but you can't have any alcohol,order some food or get out"

Said chillby with slight hostility in his rough voice

"See ya bitch,sans you coming?"

"Eh,why not"

It surprised you that he agreed after his little meltdown but you ignored the weird change in attitude and walked out with him,while you were walking down the street you both wandered into some kind of dark alley,at least,that's what you thought,really sans had just watched you stupidly walk into the obvious trap while he grinned at your obliviousness,you turned around,confused,when alphys suddenly appeared next to him,then it all clicked

"So you say you think she may not be from our universe?"

"Yup,I say we get rid of her,despite her nice attitude toward boss I still think she's a danger to us all"

"Sans what the fuck"

"Sorry kid,your time is up"

"Well I'll be damned,finally,it took longer than I expected to be honest"

"Wai-What"

"This is underfell,one of my favourite au's and you took this long to try to kill me,I'm disappointed"

"Ugh,just get her alphys"

Alphys seemed to press a button on some kind of controller and mettaton springs to life,you enter battle mode and kill him easily with your red teeth,biting him harshly right at his core,giving him an excruciatingly painful death that you found utterly hilarious,you knew this was the perfect time to start your full attack on underfell,running at sans and alphy with your purple claws and slashing them causing them to float upwards and at a clouds height,you then saw a dandelion,a beautiful flower yet called a weed and hated by all,just like you,so you turned one of your claws green and touched the flower causing it to grow at incredible speeds,you sat down,watching it branch out into vines that would cover the whole underground in a matter of hours,a few hours later once the underground was covered in dandelions and vines you sprung out a purple claw and scratched the flower causing the vines to emit a purple gas that made everyone float in the underground,immobilising them and making it look like a scene from IT,truly a wondrous sight to behold,you made yourself float and examined your fine work,amused at the screams of those still awake,the gas also mixed with the sleepy contents in the flower making the gas extremely sleep inducing so most of the residents would fall asleep,the few who didn't would just have to sit there and suffer in silence,hilarious,after an hour or so of this it eventually got boring,you were trying to figure out what to do whilst you waited for error,you didn't want to call him up because he would come to your immediate location,no,you wanted him to look for you at the end of the underground only to find you not there,you then wanted him to walk through the underground in search of you and get a full view of the destruction you had caused,you floated down to snowdinds shop looking around for anything to do,you grabbed a cinnabun and ate it quickly,you then spot a guitar and reach for that instead,grasping it and yanking it off of the wall it was perched on,you walked away from the store,guitar in hand,and then floated back up to a vine,sitting down and making yourself comfortable you began to strum and sing to the remaining residents of snowdin including sans,papyrus and yourself who you left awake,the song you sung was random and was not too shabby since you had been practicing for years,ever since you could talk actually,you began to play...

[There should be a GIF or video here. Update the app now to see it.]

You had just finished the song when you relaxed back into the vine you were perched on and accidentally dropped the guitar,but before you could reach to grab it it just floated up next to you,meh,but there was one thing you couldn't understand,why was error taking so damn long?...

Inks PoV:

"Ink! There was some kind of anomaly going on in underfell,nobody is answering to us and red sent some kind of distress call saying some kid had messed up the timeline,he didn't know who they were or where they came from but they had killed the human and toriel and flowey"

"Ugh,this is probably,no,definitely the work of error,lets go and stop him!"

Ink then proceeded to open a portal to underfell but not to the scene he was expecting,once he exited the portal he saw the underground covered in vines,monsters floating everywhere and the sound of someone playing a guitar and singing,the voice sounded young,like a child,maybe a survivor?

Ink made his way through the snowy forest towards the voice that seemed to be coming from snowdin,spotting error with a small monster child,presumably six or something (you look young for your age),I rushed into action to save the child,catching error of guard,but before I can stop him he shoves the little girl through a portal and jumps through after her,fleeing the battle,I have to save her,I have to save that little kitten...

Your POV:

"(Y/N) WHERE ARE YOU,KITTEN? ARE YOU HE-holy shit..."

You hear error's voice come from afar,your intense hearing comes in handy in situations like this one,you can hear his footsteps draw ever near and when you finally see him the look on his face is ecstatic,he rushes up to you,using his strings to pick you up and pull you to the ground

"(y/n) you cheeky thing,did you do all this?"

"*giggle* yup!"

"Heh,wow,like father like...daughter..."

Error seems to pause,obviously deep in thought and contemplating what he had just said,he was your father,wow,he had never really thought of it that way before,I mean,he had,but not in the way that he really was your father,like you took after him and everything,he was so proud,at this point even the voices in his head were proud...but that's when he came...of course it was him who had to ruin the moment,he ruined everything,can't he just let him have this,yet somehow,error still kinda loved him...