As stated in the summary- major spoilers for "Bioshock Infinite" and "Undertale" here. If you haven't played the games, then please, please, play them before reading this. It would be terrible to have them ruined for you by my scribblings. Now, with the warning out of the way, on with the fic.

It was done.

It had taken a horribly long time, or so it seemed to Elizabeth- it felt like she'd been fighting against her father's thrashing limbs for an age, as his instincts tried frantically to sabotage his sacrifice. But now, his movements had slowed, and the stream of bubbles gushing from his lips had sputtered out. It was finished.

Elizabeth looked up from Booker's icy white face as his body began to drift away from her, carried away by the current. It was a beautiful day, she thought in a distant sort of way- the sky was a deep, clear blue, with barely a wisp of cloud to break its infinite stretch, and the Sun made verdant green of the grass on the bank look almost luminous. Its rays were reflected by the river in dazzling arcs, transforming the stretch of water into shining crystal- aside from the patch where the witnesses of the baptism still stood, trapped by Elizabeth's steely grip on reality. There, the light warped and stretched, leaving the surface of the liquid dull and scummy.
She knew that sooner or later, she'd have to relinquish her death-grip on the time-space continuum and bring the frozen group of worshippers downstream back to life, but for now she kept them motionless, until she could vanish from this timeline like her alternate selves were doing even now, rippling waves of non-existence spreading out across their little group and spiriting each iteration away. Elizabeth knew she wouldn't disappear like them, fading away to nothing as the paradox they'd just caused sank its teeth into their forms. She was the one whose Siphon had been destroyed: the regular laws of time and space had no hold on her. She had no need of a father, or even a mother- or at least, in the physical sense. However, as she tried to ignore the still white form floating down stream at the corner of her vision, she admitted to herself that Booker, in the short time they'd spent together, had created a vacuum in her life, a need for someone else she could rely on. He'd always been there for her when she needed it most- complaining and sarcastic and bitter, yes, but he'd never abandon her.

She gave a huff of amusement, or at least as close to amusement as it was possible to get. She was the one with the power to make tears in the fabric of space-time, and yet he'd gone and made a great gap in her life that she knew was never going to be filled.

She felt like she was all gap now, really. Since the Siphon had been shattered by her past- and present-guardians, she'd known what she had to do for the sake of all the timelines, and had been trying to brace herself for it. She'd anticipated horror, fear, pain, grief. But she felt numb, as if Booker had managed to latch onto all her emotion and drag it out of her life along with him. She felt a faint pang of anxiety at her lack of emotion- was this a sign that she really was the strange, wild creature they'd believed her to be, her humanity spread so thinly across the plethora of Elizabeths in the multiverse that there was none left for her? However, the pang was brief, and faded into insignificance in the great vacuum that was left of her feelings. What was she going to-

"hey everyone, hey grillbz. i'll have a- darn it, wrong shortcut. never mind. sorry lady, i'll let you get on with… wait, what are you doing?"

Elizabeth turned when she heard the sound of a voice, and froze in terror at the figure that had appeared on the bank behind her.

Elizabeth had had a lot of books in her tower, and, Columbia being what it was, quite a few of them were on the topic of religion- particularly religion in the old-fashioned, bloodthirsty sense. She remembered reading something in one book about the "Grim Reaper", the skeletal angel of death that took the spirits of the departed to their final destination, be that Heaven or Hell. She didn't know where this creature would take Booker- she thought he deserved Heaven but she knew he wouldn't agree, and she wasn't really sure whether he believed in the place anyway- but she herself should have blinked out of existence a few minutes ago. Maybe the Reaper had arrived to discover why she was late for their appointment.

Or maybe he'd come to find out why she'd just sent her own father into death's embrace.
Elizabeth felt a short stab of relief at the fear she felt coursing through her now- she wasn't the emotionless shell she'd believed herself to be. However, it was soon submerged when the figure began making its way towards her, watching her with its empty eye sockets.

If Elizabeth hadn't been panicking, she might have realised that the figure's eye sockets weren't empty- they were illuminated by two pinpricks of light, that darted about as if they were pupils. She might have also realised that the figure looked rather dissimilar to the engravings of the Reaper in her books- instead of looming impressively above her, the skeleton was rather short and squat, and it wore a blue hoodie and slippers in place of the midnight cowl that featured in so many of the depictions.

The figure's "pupils" darted around the scene, taking in the crowd of believers (who were now starting to move, albeit rather slowly- Elizabeth's hold on the timestream was slipping due to her rising anxiety), the corpse twisting and turning in the current, and the terrified young woman standing in the middle of it all, her face damp with river water and tears. The left socket flared blue momentarily, but the figure gave a shake of its head and the eerie glow died away. It reached out a hand towards the young woman, who started back from it like a stray colt.

"hey, easy there, kid. i'm not gonna hurt ya- or at least, not until you've told me what's going on here."

Elizabeth stared at the bony appendage, then back at its owner, unable to take in exactly what was happening. The skeleton gave an impatient sigh.

"come on, kid. i'll admit, this looks pretty bad, but someone once told me that everyone can be a good person if they try, and while i don't know if i believe that, i'd like to. so i'm not going to do anything to you until you've told me your side of the story."

Elizabeth continued to gaze in horror for a moment, before she managed to pull herself together and grasp the hand. To her bemusement and embarrassment, a long, drawn-out raspberry noise emanated from their clasped fingers, and he skeleton gave a short chuckle (though an attentive observer might have noticed that it seemed a little forced).

"the old whoopee-cushion-in-the-hand trick, eh? never fails. i'm sans, by the way. sans the skeleton."

Elizabeth frowned in confusion at the trick: she'd read about pranks, but had never been on the receiving end of one- and certainly not at such a serious moment. Nevertheless, she nodded her head politely at the introduction.

"My name's Elizabeth. I'm- pleased to meet you."

"nice. so, how about you explain what happened here?" Sans paused. "actually, do you mind if we go somewhere else first? i know somewhere we can get something to eat and we won't have to stand in a river while you talk." He winked. "Water-bout it?"

Elizabeth blinked at the pun, but nodded, and followed the figure as he turned from the stream. As he did so, he folded the fabric of space-time around him like a complex piece of origami, making a pathway for them. With a final murmur of "Goodbye" to the best of the three father figures she'd known, she stepped through the doorway.

"burger or fries?"

Elizabeth looked at her companion with puzzlement written across her features. She could infer from her surroundings- a cramped yet cosy bar- that the items Sans had listed were probably foodstuffs, but aside from that, she really had no clue what he was talking about.
"I'm sorry?"
"which do you want, kid?"
"Sorry- I don't know what they are," she murmured apologetically, flushing. Part of her was aware that it was ridiculous to be flustered by her naivety, given what the skeleton had just witnessed her doing, but she was finding it hard enough to cling to her self-respect as it was. "Could you explain, please, Mr… Sans?"
The skeleton's grin widened- an impressive feat, given that it already stretched from absence of ear to absence of ear. "heh. kid, i'm not 'mister' anything. call me sans, okay? as for the food, a burger's a grilled meat patty, and fries are a bit like popato chisps, but thicker."
Elizabeth wondered briefly whether she should tell him that the latter description merely confused her further, but decided it wasn't worth the trouble.
"I'll have the burger, please, M- Sans."

He flashed her a thumbs-up, and placed an order with the strange being behind the counter. It was roughly man-shaped, but was completely engulfed in flame, tongues of fire constantly flickering beneath his pristine clothing and roaring in a fiery column above his head. When they had first come in, Elizabeth had had to fight down the urge to scream at the top of her lungs, certain that Sans was in fact the Grim Reaper and they'd just entered Hell, but according to Sans the flames were just a part of Grillby, and weren't even hot to touch- that is, unless the bartender wished them to be. Elizabeth made a silent resolution to never get on the silent figure's bad side.

"so, kid, now that that's done, i feel like you owe me an explanation."
Elizabeth started in her seat as she was jerked out of her thoughts, and gave a nod.
"Right." She briefly pondered where to begin. The story was so convoluted and twisted that it was hard to pinpoint exactly where it had all started. When Lutece and Comstock first launched their aerial Eden into the sky? When Booker Dewitt accrued that fateful gambling debt? Or when she'd first torn a crackling grey rip in the fabric of the universe? She sighed, and pushed her hair out of her face.
"Well, I suppose it all began when a man arrived at my tower-"

Sans listened intently to her as she recounted the events which led her to this bar. She was painfully aware that her storytelling abilities left something to be desired: she hadn't spent much time around other people, and so most of the mysteries of social interaction were a closed book to her. It didn't help that this particular story trekked its way through several timelines, paradoxes twining around one another into unfathomable knots as she tried to untangle the confusion. Their food arrived and grew cold, as she forced herself to relieve the rebellion, being captured, her time spent having her powers and her own free will leached off her inch by inch by her own father.

Finally, she reached the part where the Siphon was torn down, when she realised who her protector was and what she had to do.
"I needed to make sure that Comstock was gone- and to do that- to do that, I needed to make sure Booker Dewitt was- gone as well."
Elizabeth took a deep breath.
"So, I found the point where the timelines diverged- the baptism- and- and I- I-"
She tried to hold back her tears, and failed miserably. Everything that had happened to her over the past few months finally came crashing down on her, and she felt utterly crushed by its weight. Sans reached out a hand to her, but she flinched away, still somewhat horrified by the sight of his fleshless fingers.
"Sorry!" She shouldn't have done that. He- whoever and whatever "he" was- had never done anything to her, and she was as bad as those bigots in Columbia if she judged him solely because of his appearance.

"hey kid, it's okay. i understand what you were trying to do. but, you do know it was impossible, right?"

Elizabeth blinked, frozen mid-sob.

"What?"

"well, kid, you and i know there's infinite universes. not just a huge number of universes, but infinite universes. that means that somewhere, everything that can happen, will happen. you can make different things happen in some universes, but no matter what you do, there'll always be an infinite number of universes out there where nothing changed."
She looked up at him, a sliver of doubt threading round her thoughts.
"What are you trying to say?"
"look, i get it. i've tried to do similar things in the past. but whatever you try, there's always going to be some universes out there where your prophet is born, constructs his floating city, and takes you away from your father."

Elizabeth sucked in a breath as she realised what the skeleton was saying. She didn't want to believe him. She refused to believe him. She found the point of divergence, and stopped the paths from ever forking. She'd stopped it!
"That's not true. The multiverse is infinite, but it's made up of constants and variables. Once you find the constant you need, you can prevent it from ever happening in the first place."
Sans gave a bitter laugh.
"'constants'? kid, there are no constants. not in an infinite multiverse. if there were, it wouldn't be infinite."

Elizabeth was becoming angry now, if only to cover up the the fear welling up inside her that everything she'd done was for nothing.
"There are! I can see them! I-"
"'fraid not, kid." Elizabeth gritted her teeth at the sympathy in his tone. "some things are more likely to happen than others, but nothing happens in every single timeline. there's always gonna be some variance in events."
She stared at him, eyes bright with tears and narrowed with rage.
"So you're saying that my father's sacrifice was pointless?"
The sparks of light in Sans' sockets narrowed thoughtfully."not exactly. the guy felt he needed to atone for what he'd done to you- what other-him had done to you as well. what you both did didn't stop the timelines when he goes on to become comstock, but it did grant him some kind of peace, and let him pay off his debt to you."
"Well, how exactly can Comstock go on to become Comstock now, without being baptised?" Elizabeth folded her arms defiantly. She knew it was a childish gesture, but she didn't care any more. She was exhausted, and heart-broken, and full of uncertainties. She was owed some immaturity.
Sans sighed. "He'll still get baptised, kid, just not there. there'll be bookers that didn't make it to the baptism that day because they sprained their ankle, or got cold feet, or something else. and some of those bookers will go on to have the baptism on another day, and become comstock. sorry, but that's the way it is."

Elizabeth's mind was now a torrent of fear and fury. According to this grinning skeleton, everything she'd done- everything Booker had done- had all been for nothing. They hadn't stopped anything, except for Booker's life. Columbia would still soar above the clouds, leaving a trail of bodies in its wake, and she'd still be there to take it back down again to rain fire onto the world below. Their journey had been meaningless, and the conclusion had been even more so: it had taken away what little purpose her life had with Booker's last breath.
She couldn't let it happen. She wouldn't let it happen. She was a quantum superposition now. She had all the time in all the worlds that could possibly exist- more than enough to ensure that Comstock never surfaced in any of them.

She clenched their hands into fists.
"Then I'll deal with those Comstocks as well."
Sans' pupils narrowed to mere pin-pricks. "what d'ya mean by that, kid?"
"I'm not exactly limited by infinity any more, Sans. No matter how long it takes, I won't stop until the job is done."
"hmm." Sans looked away thoughtfully. "so that's what your life's going to be from now on, then? killing every iteration of your father, until there's none left to find?"
"If that's what it takes, then yes. I can't just ignore the problem. That's why places like Columbia exist- people prefer to deny there's anything wrong with the world if they can live in comfort."
"i see your point," Sans says carefully, toying with the ketchup bottle on the table, "but kid, that's like saying your father was only a fault that needed to be fixed. if you try killing every comstock, there's gonna be a lotta bookers who are gonna die too."
Elizabeth wished he wouldn't keep using the word "killing". "Booker already decided to accept an end rather than allow that man to keep on existing."
"one booker did. i'm not sure all the rest are gonna take to kindly to you going on a murder spree so you can- prevent murder, right? i feel like i'm missing a few key pieces of the puzzle here."
"If Booker's the man I think he was, he wouldn't let Comstock go on existing if he could prevent it!"
"he's not the sort of man who'd let the slaughter of thousands just happen, is he?"
"No!"
"how about the slaughter of millions- no, wait- billions? because it sounds like that's what you're proposing. and yes, i know you're only after comstocks, but what about all the booker dewitts who are gonna get caught in the crossfire? what about all the annas? what about everyone else who was affected by booker's actions?"

Elizabeth opened her mouth to object, and Sans' eye-lights winked into non-existence. "and even if you weren't- is this what you really want? to spend your entire existence killing, in the vague hope you could stop a killer? if so, welp- i'm not sure you're any better than your old man, kid."

Elizabeth inhaled sharply, not quite believing what he'd just said. She wasn't like Comstock. She wasn't. She wasn't doing this because of a fanatical belief in her own superiority, or because she believed she had a God-given right to do so. She was doing this because it was the right thing to do-
- but she couldn't quite ignore the fear boiling in her stomach, couldn't quite stamp down on the suspicion that the skeleton had a point. If her experiences since she left her tower had taught her anything, it was that no one was completely incorruptible- certainly not when they had as much power as she had. Even if she managed to eradicate the stains Comstock had left on the timelines without hurting anyone else, what if she decided that her job wasn't done? After all, it was only a small step from judging Comstock to judging anyone who dared raise their head above the masses in the time stream. She would start out trying to right the wrongs of each universe, but alone in her odd state of being with no others to guide her, her beliefs could all too easily become fanaticism, and if they did then there'd be no one who could stop her. If she really wanted to protect the world, Elizabeth thought, as realisation sank its claws into her soul, then perhaps the best thing she could do was leave it alone. She might have god-like powers, but she wasn't God, and she shouldn't act as if she had that authority.

She felt something wet on her cheek, and realised that she was crying.
"hey, kid."
She'd almost forgotten about Sans. She looked up, and saw him holding out a paper napkin. With a grateful nod, she took it.
"Thanks."
"don't mention it. look, i'm not saying you're evil. you're not. but you can't spend all your time trying to right wrongs that haven't even happened yet. believe me, i've tried."

Elizabeth looked up from scrubbing her face. "What happened?"
Sans gave a sigh. "it's not as epic as your story, i'm afraid. not from my point of view, anyway. it's a bit convoluted, but i suppose it started when i had a bit of an accident in the workplace at my old job. unfortunately, my old job was at the labs in hotland, and it involved unravelling the fabric of the universe and knitting it into trendy new patterns. let's just say that, at the end of my time there, i was in a bit of a unique position when it came to alternate timelines. i could see 'em all- in fact, i had to see 'em all, all day, every day. every time someone took a step, i'd see them trip and fall as well. every time someone came into a room, i'd see them go and sit down, and turn and walk away. every time someone spoke, i'd see them keep quiet. and the timeline just kept splitting, until there were thousands and millions and billions of me, all aware of each other, and all going out of their minds."

He paused, staring off into the distance as though he could see that infinite audience of alternate Sanses even now.
"it wasn't so bad, after a while. i learned to cope. but then an old friend started running experiments something called 'DETERMINATION'."
Elizabeth's brow crumpled in bemusement. "Their drive to succeed?"
Sans shook his head. "not 'determination', kid- 'DETERMINATION'. in capitals. it's basically the will to keep on going, no matter what- even if it means shredding the timeline to pieces."
Sans gave a grim chuckle. "the fabric of reality had a few creases put in it, let me tell you. one day i'm dozing off at my sentry station, same as usual, and then the entire multiverse just implodes. every single timeline just slammed together into one, all ready for someone- something- to take it apart."

Sans took the ketchup off the table and upended the bottle into his hollow mouth, the level of thick red pulp decreasing rapidly as he gulped it down. Elizabeth looked on with morbid fascination- she'd seen Booker take to drink in a score of timelines, and Sans had the exact same steely concentration in his eyes as he chugged, despite his choice of beverage. While he finished off the bottle, Elizabeth surreptitiously checked her own knowledge of the timelines, and was relieved to find it complete. It wasn't as if the events Sans was describing would have affected her- by the sound of it, Sans' own slice of the multiverse was completely disconnected from her own save for the tiny crack he'd used as a shortcut- but the idea of the infinite stretch of possibilities that had only just opened up to her shutting her out was a terrifying one. Despite the fact she'd lived without it her entire life up until now, her omniscience was a part of her, and having it cut off would feel almost worse than losing a limb.

"How did you survive?" she asked incredulously.
"kinda poorly, at first. i couldn't work out what had happened, which was something that never failed to get me mad, and what was worse was that i couldn't do anything about it." The sparks in Sans' sockets flickered and dimmed, like guttering candle flames. "but I got used to it. you can get used to anything, with time. too bad time wasn't really on my side, any more."

Author's note:

This was only supposed to be one chapter, but my love of exposition is just too great to be contained within a oneshot. I just hope that the characters didn't get lost in my sea of headcanons.