Author's Note: This story's set after the Harem Ending of an UBW/HF hybrid route. I hope I captured Shirou's analytical way of thinking about magic. Never use one word when you can use eight. 
This story's inspired by the Dungeon Keeper thread on Anime Addventure, and in particular, Pusakuronu's Dungeon Keeper Ami.
Emiya Shirou groaned as he blinked and sat up. Rin might have been experimenting with Kaleidoscope, the Second Magic, the power of alternate universes, but he didn't expect this to happen. Then again, he didn't think she did, either, given the rather surprised expression she had as he got whisked away by the prismatic magic.
Darkness greeted him. Darkness beyond that of mere night; it is the pure blackness of the grave and the deepest caves. A small pulse of magic confirmed his eyes were still fully functional; the problem was simply the lack of light. A problem easily fixed. He raised a hand, and intoned the words to activate his magic circuits.
"Trace, on."
Prana flowed through his magic circuits; their angular paths began glowing the bright red light of his magic, like a circuit board etched into his arm. He looked around; it looked like he was in a building of some sort; the walls and floors seemed to be carved from stone. Then a voice boomed from behind him, its deep tones grating like boulders grinding against each other.
"Admiring the scenery?"
Shirou's eyes widened, as he leapt to his feet and spun around. Behind him was a demon. Its skin was red as blood (though that may have been due to the light), and it stood on two cloven hooves that bore the weight of its muscular frame. It wore armor of piecemeal iron plates about its chest and hips, though what was most notable about its body was its face. Its fanged maw was topped with a piglike snout from which a heavy iron ring hung and framed with a small pair of horns jutting downwards and out to the side, while a pair of great horns unlike any he had ever seen jutted out and upwards from its forehead; they did not curve like those of a bull or ram, but rather simply extended upwards at a forty-five degree angle from the horizontal.
The thing that grabbed his attention most, however, was the scythe it carried; its blade splattered with the ancient blood of those that were slain by it. He could tell it was ancient and powerful; a cursed blade responsible for the slaughter of thousands. Men and women; adults and children; warriors and innocents. This blade did not discriminate; it simply sought to kill, much like the demon that bore it. The demon known as the Horned Reaper.
"You see, young human, we are both stuck with a bit of a problem, but now that you are here, a solution presents itself. Behind you lies a dormant Dungeon Heart; if you activate it, we can escape this prison we are trapped in."
Shirou turns to glance in the direction the demon indicates with a wave of its hand; a circular stone cap inscribed with runes of ancient evil sat there. He sensed latent and malevolent power lying beneath the seal; the power of Dark Gods and damnation.
He returned to facing the demon, and retorted, "Why should I do that? I'd never let one such as you free to kill innocents." He readied his twin swords; the black and white twins, Kanshou and Bakuya. Forged by a husband and wife team of blacksmiths from meteoric iron, they were inscribed with the symbols of Yin and Yang, and truly, they represented that cosmic force well.
The demon's face twisted into a snarl. "Fine then. You think you're a hero? It's been far to long since my scythe has tasted blood!" The beast charged with a roar; its presence building to a degree of killing power he hasn't seen since Berserker, and its footsteps igniting the floor with each step. A flash of red light, and the twin swords appear in his hands, and catch the first blow of the scythe. The shock rattles his arms; power surges down them, making them harder than steel. Even then, however, his joints still sting from stopping the force of the blow.
"I am the bone of my sword."
Shirou steps forward and to the side, stepping inside the scythe as Kanshou skids down the length of the scythe, and Bakuya swept out towards the exposed stomach of the fiend. It moved with lightning speed; the haft of the scythe spinning and knocking the blades from his hands, sending them spinning off into the darkness. The butt of the scythe slams into his chest; it sends him flying across the room. Shirou slams into the floor, rolling to his feet in an instant as he brings a new sword to bear, and recreates the blackened armor of Lancelot he had seen in Saber's memories of the fourth Holy Grail War over his reinforced clothes.
Arondight gleams with red light as the once-holy sword filled his body with its power. The demon stalked forward, shouting. "You think the heroes of the land will thank you, Dark Knight?" Shirou leapt forward, scythe meeting two-handed sword in a flurry of blows, metallic ringing echoing through the chamber as a fierce battle is fought.
"I don't need thanks. Knowing I did it is enough!" The battle lasted long enough. Kanshou and Bakuya have the power to return to their wielder, spinning as they fly through the air. They sought to return to Shirou. The demon was in the way. It grunted in surprise as the blades sunk into its back, and its defense slipped for just a moment. It was all Shirou needed. He slipped inside its guard, and thrust with the sword towards the demon's heart. He felt the blade lodge in the demon's ribcage, and then he was sent flying by the demon's kick.
"You think a blow like this will kill me? Learn your place, boy!"
Shirou rolled to his feet as he bounced once more across the dusty cobblestone floor, the skeletons of rats crunching underneath him as he bounced and slid. By the time he was risen, he had a spear in his hands; the red spear glowed as he poured mana into it to invoke its divine curse.
"Gae Bolg!"
He threw the spear, its power rending the air as it bent and broke Fate and causality. It was no longer one spear. It was an army of spears, destined to strike the demon all over its body. It was an army of spears, which had already had their barbs sunk into its body. The spears struck the demon, knocking it to its knees as Shirou ran forward, calling upon another sword.
The sword in the stone, Caliburn, appeared in his hands. The sword only kings could truly wield was at full power. Shirou invoked its name, as he smote the kneeling demon horizontally across the chest, severing its scythe arm and sending the demon as a whole crashing to the ground.
Then the demon started laughing. "Well done, mortal hero. I congratulate you on your victory. I was imprisoned in this place for a reason; if I was slain, I would simply be returned to the realm of the Dark Gods to await another summoning. Congratulations, hero! You said you would not allow me to claim more innocent lives, yet your actions have assured that I would! Remember that as you die futilely of thirst or suffocation in this prison!"
Shirou's eyes widen in shock. He hadn't considered that. He couldn't allow that to happen! He wouldn't allow that to happen! He called upon Avalon, the scabbard of Excalibur. It possessed two powers; he called upon the passive effect it granted as he plunged his hand onto the demon and released its dematerialized form inside of it.
"A healing spell? No, a copy of a healing artifact, of the light!" The demon laughed uproariously, blood spilling from its many wounds onto the tile floors in a widening pool of viscous red liquid. "You think I do not possess resistance to the magics of the light? I don't have to be affected by this if I don't want to be! You have two options, boy! First, die here, knowing you loosed a powerful demon upon the world! Second, call upon the power of the Dungeon Heart! Damn yourself, and bind me to your will! Either way, I win!"
Shirou frowned, and turned around. "That implies there is any choice at all." He strode over to the capstone covering the Dungeon Heart. "Trace, on." He analyzed the Dungeon Heart; he could not fully grasp its nature, due to the spark of life granted by the Dark Gods, but he saw enough to know how to activate it. Blood, will, and powerful magic. He could supply all three.
A flash of red, and a knife appeared in his hand. It was Dark, one of the throwing knives used by the True Assassin, and with it, he sliced open his other hand, blood splattering over the stone cover. He released the mana within it, conjuring a blade of angular crystal. It lacked an edge, but that was not what was required of it, for it was none other than the Crystal Sword of Zelretch; a key to open the lock of the Second Magic and draw endless mana from the infinite alternate universes.
He drew upon its power; mana flooded his body as the crystal sword lit up with the rainbow light of the Second Magic, Kaleidoscope, and it accomplished the impossible. He slapped his bloodied hand on the stone cover. His body became a magic circuit; his world became a haze of fiery pain as mana flowed endlessly from one hand to the other. The runes of the capstone flared with fiery light; he felt the dark mana begin to flood his body as the contract was written and signed in blood and power.
Cracks began spreading through the capstone; light shined through them from underneath as the the Dungeon Heart activated. Then, Shirou was tossed into the air as they exploded outward, the stone fragments dissolving into dust. Shirou landed heavily on the cobblestones, gazing up at the Dungeon Heart as the dust coalesced into a trio of stone arches, and pulsating red light shined up from the pit and the torches hanging from the walls around him ignited.
Shirou staggered to his feet and walked over to the lip of the newly-revealed pit, and saw a trio of staircases leading down into it, terminating just short of the pulsating mass of flesh at the bottom of the pit. It appears the Dungeon Heart isn't just a figurative name. He hears the clomp of the demon walking up behind him, but he doesn't turn around. He doesn't sense any real intent to kill. The demon's hand slaps his back hard, grasping his shoulder in a surprisingly friendly fashion for a giant killing machine.
"Magnificent, isn't it? Wealth, power, armies, women! All of them will be yours with this."
Shirou smirked. "Don't want the first three, and I already have more than enough of the fourth." Then he returned to seriousness, as he faced the demon. Its arm was reattached, and its wounds were healing. He felt some of his prana being drained away by Avalon to power the healing. "You're healing. How do I bind you?"
The demon faced the heart and smiled as he touched the heart, and Shirou felt the contract spring into place. "Like that. Tell me, Keeper. What should I call you?"
"My name is Emiya Shirou; family name first. What do I call you?"
The demon smirked; a fearsome feat with its mouth full of fangs. "Call me Horny."
This story's inspired by the Dungeon Keeper thread on Anime Addventure, and in particular, Pusakuronu's Dungeon Keeper Ami.Emiya Shirou groaned as he blinked and sat up. Rin might have been experimenting with Kaleidoscope, the Second Magic, the power of alternate universes, but he didn't expect this to happen. Then again, he didn't think she did, either, given the rather surprised expression she had as he got whisked away by the prismatic magic.
Darkness greeted him. Darkness beyond that of mere night; it is the pure blackness of the grave and the deepest caves. A small pulse of magic confirmed his eyes were still fully functional; the problem was simply the lack of light. A problem easily fixed. He raised a hand, and intoned the words to activate his magic circuits.
"Trace, on."
Prana flowed through his magic circuits; their angular paths began glowing the bright red light of his magic, like a circuit board etched into his arm. He looked around; it looked like he was in a building of some sort; the walls and floors seemed to be carved from stone. Then a voice boomed from behind him, its deep tones grating like boulders grinding against each other.
"Admiring the scenery?"
Shirou's eyes widened, as he leapt to his feet and spun around. Behind him was a demon. Its skin was red as blood (though that may have been due to the light), and it stood on two cloven hooves that bore the weight of its muscular frame. It wore armor of piecemeal iron plates about its chest and hips, though what was most notable about its body was its face. Its fanged maw was topped with a piglike snout from which a heavy iron ring hung and framed with a small pair of horns jutting downwards and out to the side, while a pair of great horns unlike any he had ever seen jutted out and upwards from its forehead; they did not curve like those of a bull or ram, but rather simply extended upwards at a forty-five degree angle from the horizontal.
The thing that grabbed his attention most, however, was the scythe it carried; its blade splattered with the ancient blood of those that were slain by it. He could tell it was ancient and powerful; a cursed blade responsible for the slaughter of thousands. Men and women; adults and children; warriors and innocents. This blade did not discriminate; it simply sought to kill, much like the demon that bore it. The demon known as the Horned Reaper.
"You see, young human, we are both stuck with a bit of a problem, but now that you are here, a solution presents itself. Behind you lies a dormant Dungeon Heart; if you activate it, we can escape this prison we are trapped in."
Shirou turns to glance in the direction the demon indicates with a wave of its hand; a circular stone cap inscribed with runes of ancient evil sat there. He sensed latent and malevolent power lying beneath the seal; the power of Dark Gods and damnation.
He returned to facing the demon, and retorted, "Why should I do that? I'd never let one such as you free to kill innocents." He readied his twin swords; the black and white twins, Kanshou and Bakuya. Forged by a husband and wife team of blacksmiths from meteoric iron, they were inscribed with the symbols of Yin and Yang, and truly, they represented that cosmic force well.
The demon's face twisted into a snarl. "Fine then. You think you're a hero? It's been far to long since my scythe has tasted blood!" The beast charged with a roar; its presence building to a degree of killing power he hasn't seen since Berserker, and its footsteps igniting the floor with each step. A flash of red light, and the twin swords appear in his hands, and catch the first blow of the scythe. The shock rattles his arms; power surges down them, making them harder than steel. Even then, however, his joints still sting from stopping the force of the blow.
"I am the bone of my sword."
Shirou steps forward and to the side, stepping inside the scythe as Kanshou skids down the length of the scythe, and Bakuya swept out towards the exposed stomach of the fiend. It moved with lightning speed; the haft of the scythe spinning and knocking the blades from his hands, sending them spinning off into the darkness. The butt of the scythe slams into his chest; it sends him flying across the room. Shirou slams into the floor, rolling to his feet in an instant as he brings a new sword to bear, and recreates the blackened armor of Lancelot he had seen in Saber's memories of the fourth Holy Grail War over his reinforced clothes.
Arondight gleams with red light as the once-holy sword filled his body with its power. The demon stalked forward, shouting. "You think the heroes of the land will thank you, Dark Knight?" Shirou leapt forward, scythe meeting two-handed sword in a flurry of blows, metallic ringing echoing through the chamber as a fierce battle is fought.
"I don't need thanks. Knowing I did it is enough!" The battle lasted long enough. Kanshou and Bakuya have the power to return to their wielder, spinning as they fly through the air. They sought to return to Shirou. The demon was in the way. It grunted in surprise as the blades sunk into its back, and its defense slipped for just a moment. It was all Shirou needed. He slipped inside its guard, and thrust with the sword towards the demon's heart. He felt the blade lodge in the demon's ribcage, and then he was sent flying by the demon's kick.
"You think a blow like this will kill me? Learn your place, boy!"
Shirou rolled to his feet as he bounced once more across the dusty cobblestone floor, the skeletons of rats crunching underneath him as he bounced and slid. By the time he was risen, he had a spear in his hands; the red spear glowed as he poured mana into it to invoke its divine curse.
"Gae Bolg!"
He threw the spear, its power rending the air as it bent and broke Fate and causality. It was no longer one spear. It was an army of spears, destined to strike the demon all over its body. It was an army of spears, which had already had their barbs sunk into its body. The spears struck the demon, knocking it to its knees as Shirou ran forward, calling upon another sword.
The sword in the stone, Caliburn, appeared in his hands. The sword only kings could truly wield was at full power. Shirou invoked its name, as he smote the kneeling demon horizontally across the chest, severing its scythe arm and sending the demon as a whole crashing to the ground.
Then the demon started laughing. "Well done, mortal hero. I congratulate you on your victory. I was imprisoned in this place for a reason; if I was slain, I would simply be returned to the realm of the Dark Gods to await another summoning. Congratulations, hero! You said you would not allow me to claim more innocent lives, yet your actions have assured that I would! Remember that as you die futilely of thirst or suffocation in this prison!"
Shirou's eyes widen in shock. He hadn't considered that. He couldn't allow that to happen! He wouldn't allow that to happen! He called upon Avalon, the scabbard of Excalibur. It possessed two powers; he called upon the passive effect it granted as he plunged his hand onto the demon and released its dematerialized form inside of it.
"A healing spell? No, a copy of a healing artifact, of the light!" The demon laughed uproariously, blood spilling from its many wounds onto the tile floors in a widening pool of viscous red liquid. "You think I do not possess resistance to the magics of the light? I don't have to be affected by this if I don't want to be! You have two options, boy! First, die here, knowing you loosed a powerful demon upon the world! Second, call upon the power of the Dungeon Heart! Damn yourself, and bind me to your will! Either way, I win!"
Shirou frowned, and turned around. "That implies there is any choice at all." He strode over to the capstone covering the Dungeon Heart. "Trace, on." He analyzed the Dungeon Heart; he could not fully grasp its nature, due to the spark of life granted by the Dark Gods, but he saw enough to know how to activate it. Blood, will, and powerful magic. He could supply all three.
A flash of red, and a knife appeared in his hand. It was Dark, one of the throwing knives used by the True Assassin, and with it, he sliced open his other hand, blood splattering over the stone cover. He released the mana within it, conjuring a blade of angular crystal. It lacked an edge, but that was not what was required of it, for it was none other than the Crystal Sword of Zelretch; a key to open the lock of the Second Magic and draw endless mana from the infinite alternate universes.
He drew upon its power; mana flooded his body as the crystal sword lit up with the rainbow light of the Second Magic, Kaleidoscope, and it accomplished the impossible. He slapped his bloodied hand on the stone cover. His body became a magic circuit; his world became a haze of fiery pain as mana flowed endlessly from one hand to the other. The runes of the capstone flared with fiery light; he felt the dark mana begin to flood his body as the contract was written and signed in blood and power.
Cracks began spreading through the capstone; light shined through them from underneath as the the Dungeon Heart activated. Then, Shirou was tossed into the air as they exploded outward, the stone fragments dissolving into dust. Shirou landed heavily on the cobblestones, gazing up at the Dungeon Heart as the dust coalesced into a trio of stone arches, and pulsating red light shined up from the pit and the torches hanging from the walls around him ignited.
Shirou staggered to his feet and walked over to the lip of the newly-revealed pit, and saw a trio of staircases leading down into it, terminating just short of the pulsating mass of flesh at the bottom of the pit. It appears the Dungeon Heart isn't just a figurative name. He hears the clomp of the demon walking up behind him, but he doesn't turn around. He doesn't sense any real intent to kill. The demon's hand slaps his back hard, grasping his shoulder in a surprisingly friendly fashion for a giant killing machine.
"Magnificent, isn't it? Wealth, power, armies, women! All of them will be yours with this."
Shirou smirked. "Don't want the first three, and I already have more than enough of the fourth." Then he returned to seriousness, as he faced the demon. Its arm was reattached, and its wounds were healing. He felt some of his prana being drained away by Avalon to power the healing. "You're healing. How do I bind you?"
The demon faced the heart and smiled as he touched the heart, and Shirou felt the contract spring into place. "Like that. Tell me, Keeper. What should I call you?"
"My name is Emiya Shirou; family name first. What do I call you?"
The demon smirked; a fearsome feat with its mouth full of fangs. "Call me Horny."