Hey! This story is part of an AU a friend of mine and I have been working on. Everything's still in the works and this is at least somewhat canon (for now) in the AU, so there's a few things I should explain. As it is, Underheart basically takes the plot of Kingdom Hearts and puts it in the Underground with Undertale characters. Asriel serves the roles of Kairi and Riku, Frisk and Sans and Papyrus serve the roles of the Trinity Trio (Sora, Donald, and Goofy), and so on. And also important for this story, Omega/Photoshop Flowey serves the role of Sephiroth (specifically here, the fight in Kingdom Hearts 1).

There's no rhyme or reason to the content I've done for this AU, so it's also important to note that this happens nowhere near the beginning of the story, it just happens to be the first thing I wrote for it. Everything's still pretty subject to change, so if something changes to contradict a part of this, I'll probably come back and edit it so that this story is right. If something happens and this fight no longer happens in the AU, I'll just leave it as is and say it no longer really happens. I hope you like it!

The journey was almost over. Frisk was so close to catching up to Asriel. Since the last time they'd met up, he'd been acting very different...and very strange. He hadn't been like himself at all. Frisk barely recognized him anymore, and he was even calling himself a different name. Asriel had goaded Frisk into a fight too, not a friendly spar like they used to, but an all out battle. It was all so strange...they weren't sure they were even dealing with Asriel anymore. Maybe he'd become someone else.

Whatever had happened to him, they were worried. Asriel was their best friend. They didn't want to lose him, and they didn't like the way he'd changed. If there was something, anything they could to help, they had to try. But they weren't even sure where to start.

Thoughts like these and more had been spinning around in Frisk's head for a long time now. It'd been some time since they had said anything (in fact, they were even being quieter than usual, if it was possible). Occasionally, Sans and Papyrus would exchange a glance over their head. The two of them were worried, too. Not just about Asriel, but Frisk as well. The ordeal head been rough on them, and the skeletons hoped Frisk would be okay in the end.

All three of them were lost in their thoughts when they nearly bumped into the dark figure in front of them. Now they knew they were there, they were all barely able to even see them. Their black coat blended in with the area around them, and a hood was pulled up so that none of them could even see their face. They even wondered if there was someone in there at all for a moment, but a few stray strands of brown hair betrayed the stranger's presence.

Frisk quickly began to apologize to them, but they quickly held up a hand to stop before more than a few words were said. They began to speak, their words low and slow.

"So, you're the human who's been running around the Underground. Greetings. I have heard much about your travels."

Frisk nodded happily. They hadn't realized anyone had been talking about them, but they'd been travelling around so much and met so many new monsters that it couldn't be surprising.

"You've become almost famous down here," the stranger continued. "Especially for your mercy. Not once have you hurt another monster down here. It is almost something to be admired, if only for your stubbornness."

Frisk blinked. They appreciated the compliment, but it seemed a little...double-edged. They wouldn't quite describe their own actions as stubborn, but...they supposed that could be accurate. Something just seemed off about the way the stranger said it.

Not noticing Frisk's reaction, or not caring, they began speaking once more. "You play by your own rules down here, and you certainly do it well. However, it will only get you so far. What if you face an opponent that can't be spared?"

They stopped to let the trio think on that for a moment. Immediately, Sans stepped forward, a dangerous look in his eyes. He didn't seem to like the implications of that sentence, but Frisk held out an arm to stop him. They stared wordlessly at the cloaked stranger, waiting for them to go on. And so they did.

"What will you do if you face a relentless killer who refuses mercy? You'll just die over and over...and with an attitude likes yours, your journey will be over. You'll never make it past someone like that with mercy."

Frisk frowned. They didn't agree. Mercy could always work, and everyone deserved to be spared. There was never a reason to attack someone. There was a lot they wanted to say, but they chose to just shake their head. It got the meaning across, at least.

"You don't agree?" The stranger laughed. Frisk could just make out a smile under their hood. "Perhaps it's best to show you, then. You can see for yourself what it's really like, how hopeless your ideals are. I can make it happen."

"whoa, there." Sans pushed past Frisk, sizing up the stranger in front of him. They were about the same height as Frisk, so he was just a bit taller. He glared down at them. "the kid's been through enough already. just leave 'em alone, okay?"

Frisk shook their head. They could take it. The three of them could do anything together.

"THE HUMAN IS RIGHT, SANS!" Papyrus had been temporarily distracted, but quickly came back to the conversation to agree with Frisk. "AS LONG AS WE STICK TOGETHER, THE POWER OF LIGHT SHALL PREVAIL! THERE'S NO NEED TO WORRY!"

"i guess you guys are right," Sans conceded. "whatever happens, we can take 'em."

The stranger laughed again. "Together? No, that won't prove anything. The human will be taking on this fight alone."

The trio shared a worried glance before the skeletons spoke again.

"ALONE? THAT DOESN'T SEEM SAFE. I DON'T LIKE THIS IDEA."

"yeah. sorry, bud, i think we're backing out of this one."

For the third time, the cloaked figure laughed. "You've misunderstood me once again. This was never you choice to make."

Frisk blinked, and suddenly they were by themself. The area they were in now was completely dark, and nothing was around them. They weren't sure how they even got here. Did the stranger do this? Was this the battle they were talking about?

They felt scared and alone. They called out to Sans and Papyrus for help. They called out for Asriel. But nobody came.

They reached out to their save point. They could still feel it, still use it. If they could SAVE, they'd feel safer. More determined. The save point appeared in front of them, and they began to SAVE.

Several cracks began to appear across the save point until it shattered apart completely. They were no longer able to save.

Now terrified, they began to run. They weren't even sure what they were running to, or what they were running from. All they knew was that they were alone and saving no longer worked. A part of them hoped they'd find Sans or Papyrus somewhere around here, but they had their doubts they would. If those two were here, they would've found them by now. Or, at least, they hoped.

Frisk wasn't sure how long they ran in the seemingly endless darkness. But just when they were sure they were completely alone, a giant figure appeared in front of them.

They screamed. The figure just smiled back.

Once the shock wore off, they got a good look at what had just shown up in front of them. Whatever kind of monster it was, this didn't seem natural. Several tubes were plugged into it at various parts. Its arms seemed to be made of cacti, and instead of legs, it had eerily human-like eyes and a mouth full of giant teeth. A television sat where its head should be, displaying the face of some kind of flower monster. Its eyes, weirdly round, glowed red. It's whole body hung in the air above them, too high up for them to attack it (not they wanted to; they didn't want to hurt a monster).

A few quiet moments passed, then it laughed loudly, right in their face. Frisk felt their determination drop.

Spinning bullet pellets began circling around them, hovering for a moment before closing in. The moment one circle disappeared, another one appeared to circle around Frisk's new location. They dodge rolled around the area, avoiding getting hit as much as possible. A bullet or two grazed their sides, but otherwise they were not hurt.

Once the pellets disappeared, they stopped to take a breather. It was a mistake. They barely had a second before the monster began attacking again. This time, it unleashed flames down on them from above. They were able to react in time to avoid most of the flames, but eventually ended up dodging from one long line of fire into another. That hurt a lot; their body was covered in burns, but they refused to give up. Not now.

They looked up at the monster to see that the television's display had changed. Instead of the flower's face, they now saw a small, aqua heart on the screen. The word "waring" flashed above it.

What did that mean?

Immediately, they were surrounded by knives. They spun around in circles, at first stay in place, but soon moving around and getting dangerously close. Frisk summoned their keyblade and knocked away the ones that got too close. A few knives the elected to dodge around instead, to prevent their arms from tiring out. They were somewhat safe for now, but they couldn't kee this up for long. They needed another strategy.

They called out for help, but nobody came. The knives continued spinning around them.

Frisk looked up at the monster once again, and noticed something had changed once again. The television's display was no longer showing the aqua-colored heart. Instead, the heart was floating up in front of the television, dimly glowing in the darkness. Perhaps they could use this to their advantage. There didn't seem to be anything else they could do, at least, so it was worth a shot.

Carefully, they weaved around the knives, avoiding getting hit until they were almost directly under the giant monster. They'd thought perhaps they could reach the heart and do something with it when they got there, but they could tell now that that was unlikely. The heart was several feet above them, and there was no way they'd be able to reach it. They reached uselessly up towards it, expecting nothing. However, they were surprised. The heart began floating downwards and landed softly in their outstretched hand. They brought their hand down and up to their chest. The heart disappeared through their sweater and a soft blue glow surrounded them for a short time.

They now realized that that hadn't just been a heart. That had been a human soul. And now it was in their possession.

Some sort of energy wave pushed Frisk back away, towards where they had been when the battle had started. The flower's face reappeared on the television's screen, now looking confused. They had gotten out of the frying pan only to go into the fire. (Almost literally, since the monster had fire attacks).

This time, they had to look up to dodge. Dozens of bullets were raining down from the air above in random patterns, exploding upon impact with the ground. They ran around to avoid them, but when something hit their side they were forced to look closer to the ground as well. Giant finger guns lined the area, firing finger bullets at them. With little choice in action, they dodge rolled around desperately, trying to avoid getting hit. They weren't completely successful, but it was all they could do. Their keyblade was useless if they couldn't block the attacks or attack. There was no time to do either.

Finally, both attacks ended at the same time. The television displayed another soul on its screen, this one orange. Frisk stopped to look up at it, taking a moment to catch their breath. This, however, was a poor choice. The mouth on the monster's lower body shot out a laser beam straight at them, causing them to take massive damage.

They couldn't keep this up for much longer, and they weren't closer to figuring out how to beat the monster without fighting...

By the time they recovered, the next attack had begun. They were surrounded by circles of giant gloves, spining around an empty center. Once again, they saw that the soul was floating in front of the television, glowing a pale orange glow. Were these souls controlling the attacks, or was the monster using them to do this? They didn't have time to think about it, and it didn't really matter anyway. The only thing that might help them now was getting that soul.

Carefully, they used the empty space in the glove circles to dodge around and get closer to the monster once more. Thankfully, the attacks used when the souls were out were a lot easier to avoid than the monster's regular attacks. Without getting even another scratch, they once more got close to the monster and reached out for the soul. The soul responded in the same way, and they were able to claim this one as well. An orange glow surrounded them, and they were pushed away once more.

The next several minutes followed the same pattern. The monster would attack, then a soul would appear and attack, Frisk would grab the soul, and rinse and repeat. The attacks of both the monster in front of them and the souls got progressively harder to avoid, and Frisk even died a few times. But thankfully when they reloaded, they still had all the souls they'd gotten before. They weren't sure if having the souls would even help, but it was some kind of progress at least, and they didn't want to start over.

Soon, they had six different souls in their possession. They assumed the cycle would repeat over again, but there was a change this time. Each of the souls appeared around them and began circling them. They almost expected an attack, but instead the souls used healing magic on them, restoring them back to perfect health, without a scratch. They didn't stop there, either. They continued to heal for awhile longer before returning to where they were. A rainbow glow appeared around them for a moment, and they felt incredible power at their finger tips, power they hadn't had before. Perhaps getting the souls had been the right move after all.

It was back to the old grind after that. The monster went back to its attacks, but even that was a little different now. The attacks were slower, weaker, and the monster seemed a bit weaker as well. Taking the souls had definitely made a difference. Perhaps now they could progress in the fight.

By now, Frisk was certain that there was no way to finish this battle without fighting. No amount of actions would change this monster's mind about battling them. But perhaps there was another way. They recalled another monster's advice about fighting...

If you whittle someone's health down enough, they might allow you to spare them.

Of course, that in itself was another problem. They couldn't reach the monster to hit it with their keyblade, and they didn't have any access to magic spells here. But maybe the souls could help with that. It was worth a try...they couldn't have imagined that feeling when the souls healed them. The souls had their own power.

Deep in their soul, Frisk reached out and tried to connect with another one of the souls. For now, their main priority was not getting hurt by the monster's attacks. Whatever soul could help them...

Something in their stomach shifted. Their soul called on the power of another. Immediately, their keyblade formed back in their hand. They could no longer move, but their reflexes were faster, sharper. They used the keyblade to block all oncoming attacks, and somehow they didn't even miss one. They were completely unharmed. They were safe for now, but that only made the battle a stalemate... They needed some kind of edge so that they could finish this without any casualties.

Perhaps the souls had something to help with that, too. They reached out to connect with a soul once more, this time their priority to do a little bit of damage to the monster, just enough to spare them...

Something in their stomach flipped. Their soul called on the power of another. The keyblade still in their hands glowed with a yellow light, brimming with magic. Frisk stared at it for a moment in surprise, their eyes wide, before carefully pointing it at the monster. A large yellow blast formed at the keyblade's tip and fired at the monster.

The monster cried out, and they could see its health drop.

At the same time, Frisk fell backwards from the recoil. A stray bullet pellet struck their face while their guard was down. They could feel their health dropping, too, but the souls they'd retrieved quickly healed them once again.

They could do this. They could win.

Once the monster recovered, it launched its next attack. Several projectiles flew towards them, quick and low to the ground. Even using the souls, Frisk couldn't block that many that fast, and they could dodge under them, and they weren't good enough at jumping to avoid them that way. They closed their eyes a third time, they needed the power of a soul that would help them avoid this attack...

Something in their stomach dropped. Their sense of balance improved incredibly, and their legs felt much stronger than usual. Using their keyblade as leverage, Frisk launched themselves up in the air, jumping to avoid the projectiles. They kept it up as long as the attack continued, only taking minor damage. Once there was another lull in the action, they relinked their soul and shot another yellow blast at the monster.

Another hit successfully was landed. They paused for a moment, waiting to see if they could spare the monster now.

No such luck. They guessed it was still too soon.

So they continued on. The monster didn't seem to have any other souls they could attack with, so this became a new pattern. The monster would attack, and Frisk would either use the souls to block it or dodge around and above it. Then when they got the chance, they'd switch to the offense and get one hit in. And then they'd repeat. Each time they got a hit in, Frisk always stopped to see if the monster could be spared yet. But they never quite got there.

After a point, it was clear they were wearing the monster down. Its attacks slowed down, and their own attacks had more and more of an impact on it, no matter how careful they were. And eventually, they held up their keyblade for one more blast, and the monster was defeated. It wailed in pain as it fell to the ground. Frisk's hand hurried to cover their mouth as they gasped in horror. They had never meant to do so much damage to it at once, they hadn't meant to win like this.

Then, they felt the familiar sensation of a save file loading. The monster was hanging up above them once more, completely unscathed. The face of a flower monster grinned down at them.

"Did you really think...just because you had all the souls, you were in control?"

Its voice was horrifying at best. The first few words were higher-pitched and quiet, almost sounding like a child was speaking. But as it continued to speak, its voice grew deeper, raspier, louder, and more malicious. When they finished the sentence, they really did sound like a monster. Not a real monster, but the monsters of human fantasies and fairytales.

"You may have used them against me, but I'm the one with power over them. You've only made things worse for yourself this way."

Frisk felt a familiar sensation, like when they called out to the other souls to use their power. But this time wasn't of their own will, they hadn't reached out at all. But still, something in their stomach lifted, and they almost felt nauseous. This wasn't a soul they had linked with before. Once it was complete, they found they couldn't move whatsoever. Their keyblade had disappeared as well, and they couldn't call it back. Not even reaching out to the other souls worked. They were completely stuck, and completely at the monster's mercy.

"The souls are mine. Your save point is mine. You don't stand a chance, you idiot. I'll keep killing you until you finally give up."

There had to be a way out of this. They were determined.

A ring of bullet pellets formed around Frisk. The monster laughed its horrible laugh once more, and the circle started to close in on them. They closed their eyes, preparing for the worst...but when the ring touched them, they didn't take damage. Instead, they were healed from all the damage they had taken in the fight.

They opened their eyes once more. The monster looked dumbfounded, its mouth agape as it tried to piece together what had just happened. Frisk wasn't sure, either. That attack should have finished them off, this was impossible. Their stomach relaxed, and they could move once more, too. Was the monster losing control over the souls? ...They reached a conclusion at the same time as the monster.

"No...no! This can't be! I'm the god of this realm, I'm supposed to win! What did you do?!" The monster's voice rose again in its panic, and it started sounding like a little kid again. At least, a kid that was losing at their favorite game.

The six souls reappeared around Frisk once more. They began to circle around the human, who was beginning to relax. It looked as though the souls would help them now, instead of the monster. They were in control. They were safe. A rainbow glow surrounded them, and they felt themself lift up into the air for a moment, before disappearing from the area completely.

Frisk blinked, and they were back where they had been before. Sans and Papyrus were just behind them, but they couldn't see that stranger anywhere. It looked like they'd vanished during the battle. Frisk wondered where they'd went, but they didn't have much energy to really think much about it.

"HUMAN! YOU'VE RETURNED!"

"whoa, kid, are you okay?"

They turned to see both skeletons rush up to them at once. Papyrus and Sans both looked them over to see if they were alright. They were tired from fighting, but didn't have a scratch on them. Smiling, they gave the skeletons two thumbs up, indicating they were fine.

Please just ignore Sans, I know he's horribly written here, but for some reason I just couldn't...write Sans properly here, in this situation. I'm sorry, I'm aware, but it's good enough because he barely does anything anyway. Just ignore him.

Honestly, a lot of this was guesswork, and how accurate this is for how things work in this AU is dubious at best. I'm not really sure how battling works in Underheart, I'm not sure if it's different or how it works in this battle specifically, I'm just not sure on a lot of things.
-Frisk may or may not be able to use the Kingdom Hearts magic spells (Fire, Blizzard, Cure, etc) but I've guessed they can
-I'm not sure if once Frisk gets the souls if they'd be able to use them in battle, but I've guessed they can
-And if they can use them in battle, I've guessed that it's like summoning or using D-Links
-I'm not sure how battling would actually be like, but I've guessed it's a mix between Undertale and Kingdom Hearts, maybe leaning towards Kingdom Hearts. As for this battle specifically, I've guessed it becomes a lot more like Undertale
-I'm not sure if Frisk's soul can be seen by them, the enemy, the player, but I'm guessing it can't. But if it could be, my headcanon is that it would always be red, even when using abilities from different soul modes, since their soul isn't just switching modes, just borrowing abilities.

Not a lot's set in stone. And I could be completely off or completely right. But like I said, if I'm wrong about something that affects this, or something changes and it affects this, I'll come back and fix things unless this just no longer happens. Anyway, none of that stuff's interesting for anyone reading this, so I hope you enjoyed this!