If Sans had a heart, it would have been hammering through his ribcage. He tore his legs out of the river, drenching the child's sweater, and leapt up. "what do you mean, kid? you need to start making sense right now!"
The roar just kept going. He could feel the emotion of it resonating in his bones: a pain more intense than he thought could possibly exist.
The child rose and began wringing the water out of his sweater. "On the first loop that I destroyed this place, he just stood there and suggested we talk it out over a cup of tea. He was like a big bottle of magic, practically asking to be cracked open and absorbed."
Sans darted his eyes about. The sound seemed to be growing more intense.
"It went like that for a long time, on mechanical repeat. But the pleasure soured after a while. I got tired. So tired."
Sans wore a nervous smile. "and here i was telling myself i had you beat for three hundred years."
"It didn't feel right anymore. I realized I didn't like the way your bones splintered, or how the ketchup you always drink spills out. So I stopped killing."
The kid knelt down and slid a pair of knives out of his sweater sleeves. He placed them on the ground and began sifting through his pockets.
"I just hid from you, or talked you down. Sometimes, when I said the wrong thing or the watch put you in a bad mood, I'd just let you kill me. It was easier that way. It felt better."
Tremors flowed through the rock beneath Sans' feet. He looked up and saw dust falling from the ceiling.
"If Asgore lives long enough, then he finds out. He learns about how I killed Mo—killed Toriel... Sometimes I think he knew from the start, and it just snaps inside of him at some point."
The child dropped two fistfuls of metal knitting needles onto the stone. They clattered like broken bells. Sans could see they had been filed down to a wicked point. The child tested their tips with his finger and began weaving them through his sweater, like throwing knives in a sheath.
"He consumes the souls, all six. Then he comes for me. No matter where I go, he always finds me."
The kid picked up the knives and rose with a slow breath.
"wait, wait. are you going to fight him? how long have you been at this?"
The child shook his head. "A long time. But it helps, believe it or not. Having a goal is important when there's nothing else to measure."
"have you beaten him like this?"
"You should go, Sans. I can tell you from personal experience, that much power makes it hard to think. He won't recognize you."
Sans shook his head so hard his neck popped. "no, i'm staying ri—"
The cave entrance exploded. Stalactites and chunks of granite hailed down on the room.
The kid sidestepped and rolled, while Sans teleported back and forth.
Light spilled out of the jagged hole in the wall. It silhouetted a hulking form.
Sans' jaw went slack.
The form was easily twelve feet tall. It stomped forward and shook the ground, leaving cracks in the stone behind each step.
"asgore?"
The king roared with such force that it nearly knocked Sans off his feet. He tried to lean into it like a hurricane wind.
Asgore stood before the pair in the tattered fragments of his armor and regalia. It had sheared apart and torn from the inside out. The six human souls formed a ring on his chest, and were embedded into it like brands.
Sans shuddered when he looked at Asgore's face. His horns had grown and branched apart. They formed a twisted, thorny mess atop his head. His eyes were pupil-less and black as ink. He bared his teeth; they were the size of shovel blades, and razor sharp.
Asgore pointed his trident at the kid. It was six feet long, but it looked like a children's toy in his hand. "YOU KILLED HER!"
He hefted the trident and threw it. It ripped through the air like an artillery round and pierced the river behind, sending a geyser of water fifteen feet into the air.
Sans could barely track it. For a second he thought the kid was gone, skewered at the bottom of the river. He did a double take, and saw the kid crouching low to the ground like a cornered animal.
Asgore slammed his fist against the cave wall and sent a few more stalactites crashing down. "YOU KILLED HER!" He lunged forward and readied his claws. They were like sharpened railroad spikes.
The kid tensed and dove as the colossal limbs converged on him. They cleaved fissures into the earth. Again, Sans thought it was over, but the kid darted under Asgore's legs and began slashing at his heel.
Asgore bellowed and attempted to stomp him to death, but the kid was already rolling out before the attack began. He drew a few needles out of his sweater and tossed them, peppering Asgore's face with puncture wounds.
This evoked yet another roar from Asgore. He barreled forward and struck at the kid with huge, sweeping blows. The kid dodged for several seconds, but was clipped by a glancing hit. It sent him cartwheeling through the air to land hard on his side. Sans could hear the crunch of bone from halfway across the room.
The kid propped himself up with his one good arm. The other hung uselessly at his side. He fished a smashed piece of butterscotch pie out of his pocket and wolfed it down in two bites.
Big, black tears collected in the corners of Asgore's eyes and he began to froth at the mouth. "SHE TRUSTED YOU!"
Asgore opened his jaws and vomited a pillar of fire. The kid scrambled to his feet and sprinted for the river. He leapt just as the flames touched him. The stream of fire seemed to go on endlessly. It clashed with the water and sent up billowing clouds of steam. By the time Asgore was finished, the river had shrunk by nearly a third and thick mist filled the room.
Sans spotted the kid emerging from the water. One arm was still mangled, and there were scorch marks all over him. He gripped his remaining knife in his good hand and gave Asgore a hard stare.
"YOU'LL PAY!" Asgore thundered over and tried to flatten the kid with his fist, like crushing a bug. But the kid was gone. He had just vanished into the mist.
Asgore began to grunt in pain as phantom slashes struck his lower back and legs. He thrashed about, hoping to land a counter-blow, but the cuts just kept coming. It went on for nearly a minute, despite the great gouges Asgore tore out of the earth with every swing.
"ENOUGH!" Asgore breathed a ring of fire around himself and began taking a deep breath. His chest swelled to almost grotesque proportions and he let out another shockwave roar. The force of it banished the mist in seconds.
Sans scanned for the kid and found him on Asgore's back. He held the dust-stained knife in his teeth and climbed with his one good arm. He balanced himself atop Asgore's shoulders and prepared to plunge the knife into the base of Asgore's neck.
The long roar had broken down into a series of heaving sobs. "Tori. Why did they take you too?"
The kid hesitated.
Asgore lurched back, and the kid lost his balance. He tumbled to the stone and Asgore whirled to face him. "WHY!"
The claws descended and the kid made no effort to dodge. They struck him, sinking down to the quick. He coughed, bringing some cherry-red to his lips.
Asgore drew closer to the child. "Why?"
The kid made a few attempts to lift his arm, and finally managed to reach up to touch Asgore's tear-streaked face. He tried to speak through the blood pooling in his mouth. "I'm—s—sorry, Dad."