I was on an Undertale kick for much of the last few months of last year, and I began writing an Undertale story where, instead of Frisk falling into the Underground, it was Illya and Kiritsugu, some time between the events of Fate/Zero and Fate/Stay Night. I also wanted to integrate the two universes in a fashion. I eventually became bored, though a lot of this eventually became my Undertale/Dragonball Z crossover Super Saiyan Saviour. One day, I may revisit this story...

JUSTICE FOR THE UNDERGROUND

CHAPTER 1:

WAY TO FALL

In a remote corner of Germany, there was a large mountain known as Mount Ebott. Legend had it that, centuries ago, monsters lived on the land in harmony with man, monsters with the intelligence of men, capable of great feats of magic. Then, war broke out between man and monster. A war that ended with the monsters sealed forever, deep underground.

But history is known to lie, and is frequently written by the winners. And legend and myth, at best, are distortions of history in any case. While the legend was true in broad strokes, it failed to capture the truth behind the legend. Yes, monsters were real, and war broke out between them and humanity. They were a different kind of monster to the Phantasmal Species, though. The closest that one could say about them were that they wielded a different kind of magic to the Magi.

One truth that had been obscured even from legend, known only to a select few, was that the monsters' downfall had come from treachery, from those who they had thought were allies. A family of Magi who had taken secrets from them to become great…and had betrayed them when it suited them, believing they had learned all they needed to from them. Another family had done the same, but they hadn't been allies of the monsters. Originally, this first family was. It was this family of Magi who created the Barrier that kept the monsters from escaping. And in locking their former allies away, this Magi family created the cruellest of locks.

Only a human could break the lock. Only a human could pass through the Barrier into the Underground, and only a combined human and monster soul could leave. And if there was one thing that this family was good at, it was magecraft of the soul.

This family was the von Einzbern family. One of the most powerful, ambitious, and cruel Magus families. A family that would think nothing of sacrificing a living, thinking being if it meant they could get what they wanted. True, many Magus families had the same attitude, but the von Einzberns took it up to another level. They were one of the three Founding Families of the Fuyuki Holy Grail Wars, and every year, they would produce the vessel that would become the Holy Grail.

The Holy Grail was not the drinking vessel that had once belonged to Christ, but rather, a magical artifact that could grant any wish, but to a single person. Not only that, but it was created as a pathway to Akasha, the Root of All Things, a noosphere of everything that was, is, and may yet be. But in order to manifest it, a vessel was needed. A living being. In all prior Grail Wars, the vessel was a Homunculus, known as a Lesser Grail.

In the most recent one, it was a Homunculus by the name of Irisviel, a woman who became something more than her purpose, even if she never fought against her fate. The reason why? Because of her husband, the man that the von Einzberns hired to win the Grail War on their behalf. A ruthless mercenary and assassin known by the sobriquet of the Magus Killer. His name was Kiritsugu Emiya. A man who wished to use the Grail to wish for what he thought was otherwise impossible. World peace, now and forever.

For such a goal, he was willing to do anything.

The Grail War involved summoning beings known as Heroic Spirits, heroes from myth, legend, and history, and having them as familiars known as Servants, fighting proxy battles. However, the Magi involved tended to be ruthless and vicious, with only a couple of exceptions. Kiritsugu was no exception, and his relationship with his Servant, Saber, aka King Arthur (who was actually a woman called Arturia: yet another distortion of history) was far from good. Kiritsugu viewed her as a tool, and her chivalry and heroism as contemptible, given the wars and atrocities heroes inspired, never allowing himself to consider that heroes did as much good as they did ill.

Kiritsugu, however, learned a horrific truth as the Grail War came to a close. The Grail was tainted by the spirit of Angra Mainyu, a Zoroastrian devil who had once been a man, long ago, before being made into a scapegoat, suffering for being the embodiment of the evils of the world. Angra Mainyu had tainted the Grail since the von Einzberns cheated in the previous Grail War. And it intended to grant Kiritsugu his wish for world peace…by wiping out humanity. Kiritsugu refused, and for his pains, was cursed, his magic crippled, and his life shaved away by increments.

In desperation to stop Angra Mainyu from being released, Kiritsugu forced Arturia to destroy the Grail, never explaining to her why. She disappeared in anguish. But the Grail managed to take one last bit of revenge: it unleashed a vile substance on Fuyuki, causing a fire that killed hundreds. Kiritsugu, desperate, found one survivor, a child stripped of his memories prior to the fire. All he had left was a name: Shirou.

Kiritsugu adopted Shirou, but he also had another child, one he wanted to retrieve, or at least see before he died. Illyasviel, his daughter with Irisviel. Half-Homunculus, and sure to become the next Lesser Grail now that he had failed.

The von Einzberns never wanted to listen to him, let him explain why he destroyed the Grail. He tried, but they refused. As far as they were concerned, he was a traitor and a wasted investment, not even worth the effort of killing given his crippled magic. Which was why he made a couple of attempts already to try and infiltrate the Einzbern Castle and retrieve Illya. But he failed, due to his crippled magic ability.

The third time, however, he got a lucky break. Illya had escaped the castle, mostly out of boredom, and had had enough of being kept in the castle. And Kiritsugu was soon hot on her heels. So too were the von Einzberns. And, by coincidence, Illya had been herded by her pursuers to Mount Ebott. Kiritsugu managed to intercept her…but they both ended up falling into a hole, leading deep beneath the ground…

…And this was where the story truly began…

Once, in an attempt to try and coax Kiritsugu into holding his daughter, Irisviel said that Illya had her father's eyes. In truth, she looked very much like a younger version of her mother. She had the albino looks of a von Einzbern Homunculus, particularly those modelled on Justeaze von Einzbern, the Homunculus who became the core of the Holy Grail. Pale skin, silvery-white hair, blood red eyes, and an inhumanly beautiful face…or at least she would should she ever reach adulthood. As it was, she was instead rather cute…and the sad truth was, given how she was both half-Homunculus and had been modified over her short life, she would never reach adulthood, at least in terms of physical growth.

With a soft groan, Illya roused herself from unconsciousness, finding herself on top of something rather soft. She realised that she had landed on top of her father, though while unconscious, he didn't seem injured from either his own landing or cushioning Illya's own. His handsome, albeit stubble-marred features seemed more haggard than Illya remembered, his hair even messier than before.

She fought back a scowl when she remembered what Old Man Acht, AKA Jubstacheit von Einzbern, had told her. He claimed that her father had betrayed the von Einzberns, and had replaced Illya with some no-name orphan boy. For a moment, her anger was on her father…until she realised that Acht had said that Kiritsugu didn't care about her anymore, hadn't come back for her, and never would. If not…why was he here? Some of the anger turned onto Old Man Acht…but some still remained directed at her father.

After all, Acht had told her who her father really was: a mercenary, an assassin, the infamous Magus Killer. And while a Magus walked hand in hand with death every time they embarked on magecraft research, Acht seemed to imply that her father was a soulless, heartless monster and a hypocrite, a mercenary who had deliberately betrayed the von Einzberns in the task he was paid to perform.

Illya couldn't reconcile that with the game she used to play with Kiritsugu, of counting walnut buds in the snow. Of the love she knew he showed her. And the fact that he was here and now…having tried to cushion her fall with his own body.

She looked around the cavern they were in, getting up, feeling a throb from her bruised body. It had a dim but gentle light, and it seemed they had landed in some sort of underground garden. Yellow flowers were everywhere. It was an oddly beautiful place, she felt. A stone passageway led off into darkness, where she could vaguely see a stone archway.

An impish notion came over Illya's mind. Maybe she had ended up falling down a rabbit hole, like Alice, and had ended up in Wonderland. But then, that line of thought was brutally derailed when she remembered where she was.

She knew the legends about Mount Ebott. She knew it was where monsters had been sealed by her family after the great war, to stop any from breaking out again. She didn't know there were ulterior motives, but that wasn't the point. The point was that she realised that she was currently in a cave that may be linked to the subterranean civilisation of the monsters her family imprisoned. Her family. And she doubted that any monster would be glad to see a von Einzbern.

Not unless revenge was on the table, anyway.

Illya, realising she might need all the help she could get, turned her attention back to her father. She had some small knowledge of first aid and medicine: all von Einzberns did, all the better to create Homunculi. After checking over her father, she realised he was badly concussed, and might remain unconscious for some time yet. She cursed quietly to herself. She knew some healing magic, but she wasn't sure she could heal this type of injury. Either a more experienced Magus, or else a hospital, would be a better choice.

Then, from the passage, she heard footsteps. The faint clacking of nails or claws on tiles. In a panic, Illya was about to drag her father away from where he had fallen when she realised that to move him without being careful could cause some sort of complication. She cursed herself. She was still too attached to this man who was a killer, an assassin, a mercenary who had gone against his employers.

My father. I can't abandon my father. Acht was wrong, he didn't abandon me, I can't abandon him, I can't, I can't, I can't…

Illya fought down her panic. She may be young, but she was already versed in more than a few magecraft abilities that could be used against any aggressors. Whatever was coming might be vicious, it might be too much for her…but she was damned if she was going to die without a fight. As she readied herself to fight, she saw that the monster was emerging from the darkness, and into the dim light of the cave. She tensed…until she saw the monster.

The thing that emerged could only be described as some kind of goat woman. She was tall, dressed in a tabard with an elaborate sigil on it, consisting of three triangles and a winged symbol. Her face was gentle, with a long muzzle, floppy ears, and a coating of white fur. Two short horns poked up from her head, while a couple of fangs poked out of her mouth in a manner that was more endearing than sinister.

And the moment she saw them properly, her hands flew to her mouth in shock. "Two humans? A girl, and a man…are you hurt, my child?"

"I'm not your child!" Illya retorted, in a somewhat petulant manner. "Stay away!"

The monster held up her hands in a placatory gesture. "I mean you no harm. I am Toriel, the caretaker of these Ruins. I patrol here each day to see if any humans have fallen down. To have two fall down is unheard of until now. That man…is he your father?"

"…Yes."

"He is hurt. Concussed. I can help him."

"Stay away from him!" Illya snapped.

"Don't be stubborn, child! Your father is injured, he needs help!"

"You want to kill him! You want to kill humans in revenge for being imprisoned, don't you?!"

Toriel blinked, before she said, "No. Not I, anyway. Child, I want to help you. Because if I truly wanted to kill you…" Suddenly, with a gesture, a fireball screamed through the air from the monster's hand, singing Illya's hair. "There'd be little you could do about it."

The casual display of pyrokinesis actually shocked Illya, and she didn't realise that the monster had approached them until she saw Toriel pressing her hands onto Kiritsugu's head. A green glow gently enveloped his head. After a few seconds, Toriel said, quietly, "The damage to his head has been mended. But he will remain unconscious for some time." With that, she took Kiritsugu up in her arms, and said, "Please, follow me, child."

"Illya. I am Illyasviel von…I mean, Illyasviel Emiya."

Toriel seemed to notice her slip-up, but all she did was narrow her eyes in thoughtfulness. But she said little on the way back, save for giving her directions while they made their way through the Ruins. Still, Illya had to wonder, what did Toriel have to gain by helping them?

CHAPTER 1 ANNOTATIONS:

Bit of a slow start, but here you go. Instead of Frisk, it's Illya and Kiritsugu who've ended up in the Underground.

No numbered annotations this time.