Bruce browsed the Library until he felt he'd learned as much about monster biology, history, and life in the Underground as he could. He was somewhat uncomfortable with the casual and sometimes oddly personal styles of writing in the pages. Some books were collections of completed and marked schoolwork. Some were journals or memoirs. There was also something he didn't expect. It was in a simple cipher, symbols replacing letters and other characters. It was a fascinating read on the conversion of geothermal energies in Hotland into magical electricity through the CORE. He didn't understand much of the magical part of the science behind it, but it proved that there truly was some scientific structure to magic that could be analyzed, understood. It was rare for anything in the library to have official publishers, and a lot of it was handwritten. Because of this, there were gaps.
He read about Home, and New Home, how the geography of the Underground was structured. He came from the Ruins of Home, and was headed to New Home, where the exit for him lay. He'd traveled through Snowdin and had seen Waterfall. Between Waterfall and New Home was Hotland, and a building called the CORE, which is what provided electricity to the entire realm.
What Undyne said about the war, the war between humans and monsters, it was true by all accounts he'd seen in the library. First hand documentation on aged pages, carefully preserved through potentially magical means. Recountings of the decrees the King made, of how monsterkind had reacted. The dust in the air as hundreds of them were slaughtered. Humans attacked first. It came as a surprise to the few writers who lived to pen the tale. One important fact stood out strongly to him. The cause of the war. Monsters had the ability to take a human's soul and become something much more powerful, a creature that did not exist in recorded history.
Seven human souls and King ASGORE will become a god.
There was nearly nothing about what the world was like before the war, other than "peaceful." He suspected that the pre-war history had been idealized in light of the horrors that came after. It wasn't out of the question that some of the history he'd read was embellished or fabricated. Despite this, Batman left the library with important knowledge.
News of Undyne's absence spread gradually in the underground. No one saw what happened, but it was clear that she was missing. Rumors and speculation began to spring up. The Guard was on high alert. Batman briefly considered turning himself in...
...but no. They would kill him. He would never escape. Dying here wasn't an option. He had to keep going. He couldn't abandon Gotham, after all.
There was no going back from what he'd done and no atoning for it.
He would have to go onward.
He encountered a few monsters, but he ignored them and walked on. Aaron, a flirtatious seahorse. Thrived on attention. Batman didn't give him any. Moldbygg, a towering slime monster. Wanted its personal space respected. He respected it. Shyren, who was too shy to approach. He didn't step into her space. There was a dummy of cloth and stuffing that seemed suspicious, possibly bugged or magic, so he avoided it. Considering how the first one he'd encountered flew away, it may have been the same one.
Batman kept to the dark, off the path, weaving through shadows, flitting past echo flowers without stopping to listen. It was better if he avoided as many people as possible. It was better that he left quickly.
He thought about Asgore, the king. Asgore, the one Toriel worried would kill him. Asgore, who was collecting human souls. If he'd already collected and absorbed six, he could already be unfathomably powerful. Logically, he would be waiting at the most accessible entry and exit point of the Underground. Toriel only had two requests of Batman. That he called her and that he didn't hurt anyone. He'd already failed the second, but perhaps it was time to do something about the first.
Once he was certain that he was secluded and no echo flowers were within earshot, he dialed her number, the digits pressing down easily despite the phone's apparent age. It wasn't the glossy touchscreen of a smartphone, but it was kept it good working condition, and must have been repaired at least once.
Nobody picked up. So, he left her a message and pocketed the phone.
And moved forward.
When the phone did ring, he expected that she'd called to answers his message. He was most absolutely not expecting the voice of a very enthusiastic skeleton on the other end.
"HELLO! IT IS I, THE GREAT PAPYRUS!"
"What? How did you get this number?" Did Papyrus know Toriel? Why would he be the one calling? What was going on here?
"OH, HOW I FOUND YOUR NUMBER? THAT'S EASY! I JUST DIALED EVERY NUMBER SEQUENTIALLY UNTIL I GOT YOURS! I'M GLAD I FINALLY GOT THE RIGHT ONE. HELLO MISTER SCIENTIST. I HAVE ASKED EVERYONE I KNOW TO NO ANSWER, EXCEPT FOR YOU, BECAUSE I DO NOT REALLY KNOW YOU. YOU SEE, UNDYNE AND I HAVE A COOKING CLASS SCHEDULED FOR TODAY, AND SHE IS NEVER LATE. BECAUSE THIS COOKING CLASS IS AT HER HOUSE. AND SHE IS AT HOME WHEN WE START IT. HAVE YOU SEE HER ANYWHERE?"
There was a pause, but it wasn't long enough for Batman to answer.
"OH, IF YOU HAVEN'T MET HER, SHE IS A LOUD, EXUBERANT, AND VERY STRONG FISH WOMAN. WHO HATES HUMANS. A LOT. IN FACT, IF SHE WERE TO EVER SEE A HUMAN SHE WOULD... PROBABLY BE. A LITTLE MURDERY. BY WHICH I MEAN VERY MURDERY. UNDYNE DOESN'T DO ANYTHING BY HALF-MEASURES. I BELIEVE, IF SHE WERE TO EVER MEET A HUMAN, SHE WOULD MOST ASSUREDLY CAPTURE THEM. ...IN A POSSIBLY MURDERY WAY. I SUPPOSE. IT WOULD ONLY BE NATURAL FOR A HUMAN. TO DEFEND THEMSELF. IF IT EVER CAME TO THAT. BUT! YOU DO NOT NEED TO WORRY ABOUT THAT! BECAUSE YOU ARE A BAT. WHO IS ALSO A SCIENTIST. YOU PROBABLY DO NOT EVEN KNOW WHAT A HUMAN LOOKS LIKE, HA HA. BUT UM. ANYWAYS. HAVE YOU SEEN UNDYNE? AT ALL? PLEASE?"
Batman froze.
Papyrus knew. He must have known. Why else would he have said all that? Did he knew where he was? Why was the skeleton actually calling? Dialed every number sequentially— could he have really done that? If not, where did he get the number...?
He couldn't formulate a response before the skeleton spoke again.
"IT'S ALRIGHT IF YOU DON'T KNOW. I'M JUST WORRIED. I KNOW SHE CAN TAKE CARE OF HERSELF. AND SHE IS VERY, VERY STRONG. SHE IS SO STRONG THAT SHE COULD HOLD UP THE ENTIRE UNDERGROUND WITH HER BIG, STRONG ARMS." The skeleton rambled, trying to fill in the silence. "EVEN THOUGH MAYBE, SHE DOESN'T NEED TO. BUT. IF YOU SEE HER. COULD YOU CALL ME? YOU HAVE MY NUMBER NOW."
There was a long silence. Batman didn't know what to say. Could he tell Papyrus that he killed Undyne? What would he do then?
Papyrus hung up.
Batman ran. He couldn't risk getting caught. He couldn't risk getting found.
His heart and eyes burned with guilt, shame, desperation. Did Papyrus know Batman killed his friend? How could he not, after all that? He stuck to the shadows, silent despite his heart pounding in his ears.
He killed Undyne. The consequences were coming.
The cool dark of Waterfall bled into the searing heat of Hotland. Magma flowed far beneath. In the distance, he could see a towering structure of metal, the CORE, which powered the entire Underground.
It stood between him and his destination.
The hiding places in the volcanic heart of the Underground were few and far between. Light came from beneath in a perpetual orange-red glow. Bridges of stone and earth were suspended high above the molten rock, enough so that the convection wouldn't be life-threatening. Nearly everything was exposed to sight, and the ceiling was so far above he would not have been able to grapple to it, even if he had his grappling hook.
There was only one path between Waterfall and Hotland, and on that path was a snow-covered sentry station. Oddly enough, it was unmanned. Nobody was pursuing him yet, but Batman knew that calls could be traced, and hence there was a chance that someone, possibly Papyrus, was after him. The station wouldn't provide much cover, and the snow on top of it was probably there because of magic, as he had no idea how else snow would remain in this heat.
A water cooler a few meters away from the sentry station, and further from that, a clearly marked LAB. In the distance to the left, a guarded metal spire, possibly for an elevator, with two heavily armored knights stationed in front of it.
It was possible someone could be hiding in wait, knowing that Batman would inevitably cross, but he couldn't let that stop him. He would have to be prepared for confrontation, but also be careful not to actually strike any attackers directly. He had no way to tell if any attack would be lethal, considering what he'd learned of monster physiology.
So he walked. Carefully. He listened out for sounds of movement, paid attention to the flow of air. He would be ready for any ambush.
No ambush was forthcoming.
He reached the Lab without incident, avoiding the elevator shaft and consequently the detection of the guards.
As he had no way to travel across lava, his options were limited. He decided to scale the laboratory building to get a better view of what lay ahead of him and avoid detection.
Footing was practically nonexistent, but he could just barely grasp hold between the plates of metal that made up the walls.
The roof of the lab was flat, and from it, he could see what lay on the other side. Conveyor belts stretched over the expanse, connecting outcrops of stone rising from the lava. Monsters roamed the path ahead, including... A living airplane? There was an airplane with a hat and bow. It flew seemingly of its own accord. It - she - was far too small to carry human passengers within, not that Batman thought it would be safe to do so.
Magic knew no bounds, it seemed.
Where Snowdin had a forest Batman could weave in and out of at his leisure, and Waterfall had numerous caves and crevices to travel other than the main pathways, Hotland had no alternative routes Batman could see. The only platform was the bridge of stone and machinery that lay before him. Ahead of that, he could see the elevator shafts embedded into a cliff face in front of the CORE, connected to form a sort of elevator grid. His destination would be the third floor.
He dismounted the lab from the other side and continued on.
The conveyor belts didn't seem to serve any practical purpose, moving people and objects from one rock to another. They weren't part of any combined machinery, and Batman thought bridges would have worked just fine. He didn't like the ground shifting beneath him while he walked.
Short, lava-filled rock monsters ambled about. Batman did not want to see what they could do, and he avoided them. Tsunderplane, the living airplane, snubbed him, raising her nose upon seeing him with an audible "hmph." Guess he wasn't her type.
There were vents with arrows printed on them, blowing hot air in the directions those arrows pointed. The gusts of wind were strong, possibly enough to propel some of the smaller monsters he'd seen across the gaps. He wasn't sure about the jumping capabilities of most monsters, but suspected that these vents were used by some to traverse the short gaps in the bridge. ...Even then, they seemed unsafe, and relatively useless. Perhaps they had an alternate, probably magical, means of travel, though he could imagine a younger Robin using them to assist in acrobatic maneuvers.
As it stood, he had no difficulty jumping across the gaps, and was too heavy to be particularly propelled or impeded by the vents.
Then he found the lasers. There was a bridge of metal and piping spanning from one rock to another, with laser beam emitters on a wall on one side. Orange lasers were stationary, spaced apart in wide intervals. Blue lasers scanned back and forth, sliding from side to side above the orange ones.
It wasn't his first time traversing a laser grid, though unlike with lasers he was familiar with, these appeared as solid lines with no need to be shown using mist or fog, the beams themselves glowing. He pondered their construction and purpose as he moved around them. At the end, another gap, then a cave entrance in the wall.
The area up ahead was dark, where the stone of the cave blocked out the lava's glow. He could see the glowing arrows that indicated the directions of the vents, but little else. He checked his footing and walked carefully, aiming for the vents when jumping, as those were the only placed he knew there would be footing. He tested each step before he took it, and eventually found a larger platform of solid ground.
Solid ground meant even less light, as there were no gaps for light to seep through. Though traveling in the dark was nothing new for Batman, he'd grown accustomed to having night vision goggles or a light with him in situations like these.
He crept along carefully making sure there was nothing to trip on until -
"WELCOME, BEAUTIES AND GENTLEBEAUTIES!" A robotic voice sounded from right in front of him.
Suddenly lights around the cave turned on, revealing Batman and a large square robot balancing on a single wheel in... A kitchen? The robot held a microphone up to its face, a panel of red and yellow square lights.
"IT'S TIME FOR... COOKING WITH A KILLER ROBOT!"
Canned applause sounded around as lights flashed and cameras trained their gazes upon him.
"I'm your host, Mettaton, and today we have a very special guest with us for the program!" The robot turned to Batman and held up the microphone up to his face. "Why don't you introduce yourself to the audience, darling?"
"What is this?!" Batman was very much surprised, yet something about this felt all-too familiar. The theatrics reminded him of some of the villains in Gotham. Joker particularly had a penchant for hijacking television.
Mettaton wheeled over to Batman's other side, still holding the mic up to Batman's face. "Why, it's a cooking show, and you're our guest star! Smile, you're on television!"
Batman did not smile. What he did do was look for an exit. Instead, what he found was a fully-furnished kitchen surrounded by forcefields. Even the ceiling was covered. It was an effective trap. It seemed someone knew where he was going and was intent on stopping him.
Mettaton brought the microphone back to himself. "Today's episode is very special, folks! Our esteemed guest has two choices ahead of him."
The camera and spotlights focused on the stovetop, on which a frying pan lay.
"You can fry,"
The robot spun around, suddenly brandishing a flamethrower, firing it into the air for effect.
"Or you can fry. So! What's it gonna be?"
Smells like something's cooking.