DISCLAIMER: I own neither Undertale nor Five Nights At Freddy's.
A/N: I didn't really explain it in the story, but I'm going by the ideas that the animatronics have souls and that Freddy's gets a weaker version of the Barrier that forms around it when the five animatronics are awake and moving around. That's why they couldn't get in to help until 6am.
Also, possible trigger warning near the end; being held down, helpless+alone, etc.
"Legends tell of a haunted restaurant at the base of the mountain nearby our village. In its prime, it was a place of fun entertainment, a place that parents could take their children without worry of playtime danger. However... everything changed when (the fire nation attacked) one of the children got seriously injured.
"This incident was called the Bite of '87.
"In some way not known to the general public, a child had been lifted up to the mouth of one of the animatronics while it was singing a prerecorded song. When it finished singing, its mouth closed on the child's head, taking a large chunk out of their frontal lobe, and sending them to the hospital in a coma.
"Later on, another incident related to the restaurant occurred. A man who had been recently fired from the establishment returned in a purple suit, and enticed five children into the spare parts room. Each child was stuffed into a different suit, and left to rot...
"Patrons and customers complained of odd smells coming from the animatronics, of thick red mucous-like liquid leaking from the joints, and soon the restaurant was shut down. It was left there, never sold or altered, and became an abandoned building. However, if you listen closely, sometimes you can hear the animatronics singing and playing their songs as though nothing ever changed..."
As you told the legend of Freddy Fazbear's, your audience was held captive. Intense stares, bodies leaned forward, the silence of utterly focused attention, all centered upon you as you spoke and even acted out certain parts of the story.
"Over the years, these noises have attracted many to the site, especially the teens of the town. In fact, they even created a challenge based on some prerecorded tapes found in storage, intended for new night shift employees. This challenge is not intended for the weak of heart... for they say that people have died attempting it.
"You see, the animatronics have a 'free roam' setting that allows them to move in the nighttime. Officially, this is to prevent their servos from locking up, but unofficially, it's said they couldn't be stopped if you tried. This free roam mode allows them full reign of the restaurant, but not one step outside of it.
"The challenge- nay, the tradition- created with those tapes and animatronics, is simple. The one sent there is to pretend to be a new night watchman, watching over the animatronics, without leaving for five days and five nights.
"The name of this challenge... is Five Nights at Freddy's."
By the time you were finished, they seemed slightly dazed- even Mettaton nodded approvingly. Abruptly, Undyne jumped up with an intense grin, immediately shouting, "ALRIGHT PUNK! WHERE IS THIS PLACE? CUZ I'M CHALLENGING YOU TO LAST FIVE NIGHTS AT FREDDY'S!"
You blink in surprise, leaning back with a finger pointed at yourself. The surprise is slowly replaced with a smile as you lean forward and lower your hand. "But Undyne, aren't I too soft and wimpy to do it?"
The fish woman laughed, slapping your back enthusiastically. "Punk, if you do this challenge, I'll HAVE TO admit you're not a wimp!"
The others in attendance, Sans, Papyrus, Mettaton, and Alphys, all give various indications of surprise. Sans speaks up with what they're all thinking, a cheesy grin on his face as he digs you a verbal grave. "undyne doesn't do that often kiddo. 'sides, there probably isnt any real threat there. when do ya wanna do the challenge?"
It looks like you weren't going to get out of it, so with a dramatic sigh, you reply that Monday is the best day to start the challenge. Five days and nights after all. But, you request- no, demand- the right to be in a phone call with the group during the whole time. You're totally not scared. And snacks. Loads of snacks are necessary for a challenge of this magnitude.
They give in easily- almost too easily- and you can see the amusement on their faces. Monday, being tomorrow, meant that you had the rest of the night to prepare. After winding down the night with your friends, you escaped out the door with Papyrus, heading home to get ready.
"HUMAN, WILL YOU BE SAFE DURING THIS CHALLENGE? IT SOUNDS DANGEROUS." His concern made you smile, and you waved your hand in dismissal.
"It's only legends, Paps. Don't worry, I'll be fine. The most likely thing I'll die from is boredom when nothing happens." You laughed, but noticed that he still seemed concerned, so you invited him in.
"Hey, if you're that worried, you can help me pack!" He seemed to brighten up at that, and nodded, coming right in.
"HUMAN, I WILL MAKE CERTAIN YOUR CHALLENGE IS THE EASIEST CHALLENGE UNDYNE HAS EVER CHALLENGED!" You giggled at the redundancy, and accepted.
The group dropped you off with a large duffel bag of all sorts of things. Papyrus packed a lot of his good luck spaghetti, Alphys gave you an upgraded phone with extreme battery life, Undyne lent you one of her spears, Sans gave you a bunch of assorted food and a lot of ketchup (he knew his brother's spaghetti wasn't the best option for extended digestive health), and Mettaton gave you a camera and wearable microphone for recording purposes. You yourself had packed a tablet and some books in optimism, and a cute Puppet plushie for luck.
It was sunset, roughly 8pm, and that gave you a few hours to explore. The group hung around outside mostly, working on getting the generators up for an 'authentic experience', and you waved at them from the security office window when you found it. The window was too vertically small to climb through, but horizontally wide enough that the others could watch you all at once if they wanted to.
To settle in, you spread your stuff around and sat in the dusty- but surprisingly comfortable- swivel chair. Standing, you brushed off the chair of dust, and sat back down, picking up a book from your bag and a container of spaghetti. Somehow Alphys had rigged the containers to keep the food warm for extended time, which was something she'd been working on in her spare time with the humans. It worked quite nicely- you enjoyed some warm and indescribable spaghetti before ringing up the group outside.
"Hey, it's almost midnight guys. You ready to watch me own this challenge?" Big words, considering that you were beginning to feel, perhaps, the tiniest bit of fear. Closing your eyes, you took a deep breath, and put away the book to the sounds of affirmations on the other line.
"Darling, remember to turn on the camera~" The sound of Mettaton's sultry voice makes you laugh as you do so, turning on the microphone as well.
"Is that good, Mettabae?" You hoped your teasing would flustered him- and pouted when he laughed, your endeavor not being successful.
"HEY PUNK! Maybe you should focus on the challenge now instead of FLIRTING!" Undyne's voice was loud enough for you to hear it both through the phone and the thick window, making you hold it away from your ear. You laughed, setting the phone on the table and picking up the security camera screen in its place.
12am... 1am... The hour passed quickly, with you flicking through the screens in idle boredom. In the beginning, you'd been cautious and wound up, but now you were nice and relaxed... However, seeing one of the screens removed that relaxed feeling in an instant.
Bonnie was missing.
"G-Guys... this is not the time to be pranking me..." You spoke into your phone cautiously, flicking through the screens... Bonnie was in the party room.
"Pranking you? Darling, Sans is fast asleep, and he's only one here who would prank you right now~" Mettaton didn't help your fears at all... instead, confirming them.
With a gasp, you checked your power- somehow it was at 87%... from the legends of those who survived this challenge, the lowest percentage for survival at 1am was recorded at 83%. If you kept going the way you were, you would do just fine. There was no confirmation that the animatronics would actually kill you, right?
You wished that you'd researched it before doing this.
"Guys... B-Bonnie is actually moving around..." The camera goes out in the party room and you shudder violently, flipping through all the cameras... Bonnie moved to the hallway. It was approaching... but you had one of Undyne's spears, and if there was any real trouble, your friends would come in faster than you could scream. You trusted them.
With a shock, you realized that you forgot about the door lights. There were two buttons on either side of the security camera panel, one for each of the left and right lights and doors, and you hurriedly flicked the left light. An embarassing squeal escaped your lips as you slammed the door button, closing the door on Bonnie...
They moved. They actually moved around. But it was okay. Your friends were behind you, watching you through the window. In fact, you could hear their concerned voices through the thick, dirty glass... what happened to your phone?
You glanced up from the cameras and at your phone screen, face going pale when you saw 'no connection' where the signal bar should be. The call had ended... but your friends were still outside. They could still see you. They could get you out, if you just survived.
Glancing back, you saw the concerned faces of your friends. Sans appeared to be sweating from effort, and his form was covered in blue light, as though he were trying to teleport in... oh god, did that mean he couldn't get in? Were you stuck in here until the end of all five nights?
You stared at the camera, flicking the lights and opening the left door when no shadow was cast behind it. Struggling not to panic, you tried to compose yourself- after all, you'd faced worse, right? Some creepy old animatronics weren't gonna be enough to take you down, right? You could survive five nights... right...?
You squealed when Freddy stared at you in the camera, gasping in a few tense breaths as you switched the camera to Pirate's Cove. Wait. The curtain was open. YOU FORGOT ABOUT FOXY.
Slamming the door buttons closed, you frantically searched through all the cameras, before seeing Foxy sprinting down the left hallway.
"Swiggity swooty, he comin' for da booty..." You glance at your own rear end for a second, before opening the doors and flicking through the cameras again. Behind you, the sound of dim thuds was heard, and something shook the walls of the building, dust and a loose tile falling from the ceiling... You glanced behind you frantically, trying to see what your friends were doing. Sans was aiming so blaster towards the window... you shook your head and returned to the cameras.
The power was at 10%... The time was 5am... You only had one more hour to go for this night, a fact that made you breathe a sigh of relief. Relief felt, perhaps, too soon. Chica soon appeared in the right lights, and you slammed that door shut... Bonnie shortly after appeared in the left door lights, and you slammed that door shut... both doors used up a considerable amount of power to keep going.
A soft whimper escaped your lips as the lights went out. Padded footsteps were heard approaching the left door, and you stared in fear at it... jumping in your seat as Freddy's eyes and teeth lit up. His music box was both peaceful and terrifying, like a harbinger of death, and you listened in stiff terror. You couldn't move...
The sound of Undyne shouting through the window made its way through your frozen stupor, and you grabbed the spear she'd lent you. It seemed as though your movement caused the music box to stop, and you swung out ahead of you by sheer reflex... connecting heavily with something before you.
The shrill screech of metal and a pained voice made its way to your ears. You ignored it, screaming in your own terror, as rough. metallic arms clasped tightly around your middle and pinned you to the chair. That was why they jumped at you, to pin you down so you couldn't move, so you couldn't fight back until they were stuffing you in the suit.
WAS THAT REAL TOO?
In a panicked haze, you struggled against the animatronic limbs holding you down, the swivel chair squeaking against the filthy tiled floor as it was rolled towards the left door. In the back of your mind, you noted that three of the four animatronics traversed the left hallway, which meant that you'd have a lesser chance of escape.
Being pulled backwards down the hallway, you could see the faces of your friends through the window for just a second, their expressions of fear mirroring your own. They hadn't truly believed that the legends were true, and neither had you, but then again, they themselves had only been legends to humanity... In hindsight, you should've had someone with you. They had magic, you had soft human flesh and no long range skills whatsoever. These animatronics were so much stronger than you...
"SOMEBODY, PLEASE! HELP ME, HELP ME PLEEEEASE!" Your screams were high and thin, breathless with terror as you were. Your struggles were useless as you were dragged to the spare parts room, and the door closed securely behind you and the Freddy animatronic that held you to the chair. One padded, metallic hand moved up to cover your lips, silencing your cries for help, and the other moved beneath your legs, pinning them against your chest as it lifted you from the chair. Breathless little squeals and gasps were the only sounds that escaped the effective muzzle over your mouth, as you were lifted up high and brought towards the suit on the table...
Then, Freddy froze, still holding you tightly to his chest in the air. A clock chiming slowly in the distance sounded 6am, the last thing you expected to hear at this point. Whimpering softly, you dared not move, lest it wake up the animatronic holding you...
The lights came on, revealing you in your shaky predicament. Your back was braced against Freddy's upper chest, with his left hand holding your knees up tightly against your own chest. Freddy's right hand was pressed firmly against your mouth and jaw, pinning your head in an awkward position within the crook of his neck, stretching your own uncomfortably. You couldn't wiggle free upon attempt, and straightening your legs put immediate pressure upon your neck as your body slid down.
The door abruptly burst open, and you reflexively twitched, a muffled scream escaping without your volition. Heavy coughing and gasping followed the scream, as you struggled to catch your breath, your vision fuzzing out from lack of air. Your cold, shivering limbs didn't register the touch of a bony friend, until you found yourself falling and being caught in a surprisingly gentle manner. The freedom of movement had you spasming slightly as you gasped properly for air, clinging tightly to who you now saw was Papyrus.
"THERE THERE, HUMAN. THE GREAT PAPYRUS HAS YOU. YOU ARE SAFE." Like those words were a lifeline, you held tighter, shivering in a mixture of sheer relief and leftover adrenaline. You never wanted to do that again, and you never wanted to see Undyne again.
Well, not never again, but you'd certainly be giving the fish the cold shoulder for some time to follow...