...Version one...
As he walked through the cursed flame, Shirou rejected his reality. No one is actually dead, I just have to walk on to see them again. No child could understand what was happening, so Shirou, the only part of him still left, rejected reality itself. A distortion was made, and to those who died in the fire, and to all who wished to inhabit the Earth past their time. That day, a Rex Mortuorum was born.
...Time Skip...
It was always the same dream, every night. It wasn't a bad dream, per se, but it was creepy. Every night, he dreamt that he was sitting in a large, far too big, black chair, studded with extremely realistic skulls and bones. Each time, he was greeted by almost see-through people. Hundreds, maybe even thousands of ethereal, and he had looked that word up, people.
The people would move extremely fast, but always seemed to do the same things. They would run from all over, before suddenly lining up and bowing at him, and on would give him a black staff, often putting it between his legs himself when he wasn't fast enough. Then, a man after man would come and see him, often begging for something, and countless figures would come in, doing various tasks, ranging from bringing delicious looking food, to simply standing in the corner.
Every night it got slightly slower, and he could make out a little more of what was happening, but he still couldn't understand it fully.
During the day, his life was simple. Every day, he would go to school, often doing small tasks for the teachers and his fellow students, and come home after school was over. Then, he would spend an hour or two helping those around him, and sometimes he would visit the graveyard and have a snack. Then, he would come home and beg his father to teach him Magecraft.
Eventually, his persistence would pay off, and he would be the savior of those who needed it. He would be the King of the Broken.
...Time Skip...
When Shirou had finally convinced Kiritsugu to teach him Magecraft, he was expecting to learn some healing abilities, and some powerful self-improving ability. Instead, his father told him that he had a very slight affinity with everything, but a very powerful affinity with Curses, and especially Destruction. In fact, it seemed that anything that evolved the destruction of something, in any way of looking at it, would vastly increase it's power.
He was especially good with things that didn't have a physical body, which was most of magic, or binding something that didn't have a body into matter. Recently he had used that exact method to create a mildly strong curse that made whoever he wanted it to get extremely sick for a few hours. When he had asked about why, exactly, he had made the thing, Shirou had responded with something that disturbed the ex mercenary. "I want to see if I can help people, by cursing the world." he had smiled.
That was extremely strange. But, when he had asked about what he meant, Shirou had been eager to explain. He wanted to take the Mana in one area, and force it to power a giant curse, one that went around the entire world and punished those who were thinking of doing bad things with tummyaches and bad cases of the flue. Honestly, it was extremely childish, but it probably would stop most crimes. Of course, it would also probably kill hundreds of thousands of men indirectly.
That said, it was very obvious that Shirou was trying to do good, and Kiritsugu was confident that with some direction, Shirou could help the world.
...Time Skip...
When Kiritsugu dies, his soul finally understood what his son truly was. It was true that he never expected to go to Heaven, in fact, with Magecraft he was certain that he wouldn't get any afterlife. The fact that he had one showed a massive lack in his knowledge, and in fact the knowledge of the Clocktower, and thus all Magi. Still, the fact that his son was the King of the Dead stunned him.
Now, he lived inside the massive world that was his son's soul. Every day, he watched his son in the dream he had always talked about, moving incredibly slowly because his Mana was not large enough to move at an acceptable speed. He figured it out incredibly easily, mostly because he could feel Shirou's Mana, and look Shirou's soul pressing against his own.
Shirou's actual father, biologically, stood before him. "You know how to fix this?" he asked. Kiritsugu nodded, "We need to supercharge Shirou's Mana to get him up to any acceptable speeds." he explained. Shirou's father, not knowing Magecraft or how it worked, asked the obvious question. "How do you do that?" he asked.
Kiritsugu sighed, "We need to give him some of our own Mana."
...Time Skip...
When Shirou had gotten the power of all those in his Reality Marble, he had been able to speak with everyone normally. He spoke with his father, both of them. His many subjects he had never truly known about, and he had been able to move around, and spend an entire day being king. He had been able to do the one thing he had been wanting to do, see those he had failed.
The thousands of people who had died in the fire, they were all, every single one, inside of him. Each one remained, sheltered from Gaia's will, or some evil magus. He was happy, happy that he could see his father again, happy that he was able to see those who had died in the fire, and happy that he was forgiven by them. He had set up Magecraft training with his father, his second one, and Shirou felt that, for the first time in months, all was well.
When he woke up, however, he noticed a much different change. He felt incredibly energetic, powerful, like he could take over the world. He spent an entire hour running around, his breath never speeding up, and his heart never seemed to beat faster. He had felt his Magic Circuits, and was surprised to find he had almost ten times what he had had before. He spent hours doing Magecraft, and yet never felt tired, spending hundreds times more time and Mana than he had ever managed to do before. It would not be far to say that, in a single day, he had progressed more than he had in his entire life.
When he cooked, he felt a slight, affinity to it, even more than before. He felt calmer, more collected, and much stronger than he had ever felt before. Shirou decided that he liked this new feeling.
...Version one Version 1...
That night, his father explained to him about the consequences that feeling had had on his people, and together with his fathers and family, Shirou had decided to only seek out their power when he truly needed it, in serious situations. Looking at the massive crowd of slowed ghosts, moving only a fraction of the speed they should.
That day, Shirou Emiya, King of the Dead, truly became the hero he was always meant to be, even if it was only for the dead.
...Version one Version 2...
That night, Shirou learned what had caused his happiness, taking power from others. It took a while, but eventually, he came to a choice. Shirou smiled, as he finally decided. I am king, it is only natural that I take from my subjects.
...Time Skip...
Shirou Emiya, Dead Apostle Ancestor, stared at the abomination in front of him. Saber, his Servant, was a dead woman, specifically King Author, made into legend and brought back by the Holy Grail. Looks like my days just got more interesting.
...Version two...
When Shirou had gotten up, he wasn't expecting to be killed by a legendary hero named Lancer. When he was, he wasn't expecting to brought back to life. When he returned to his house, and had gotten attacked by Lancer again, he did not expect to be saved by a legendary hero, especially one that would not tell him her name. He didn't expect to then have to chase after said legendary hero and stop her from killing his classmate, especially not Rin Tohsaka, the school idol. He didn't expect to then be dragged into his own house, and have his secret crush explain his own situation to him.
He especially didn't expect this.
In front of him, stood an exact copy of his current body, though his hair was a dark crimson, instead of a light red. He word black robes, very much akin to what Archer wore. "Hey younger me, Rin. I've been summoned as Caster, but my Master kind of died, and I was wondering if you would allow me to stay at my own house..." he chuckled.
Shirou, ever the smart one, nodded slightly. "So I become you when I grow up? Do I become a hero?" he asked, excitement at the apparent promise of becoming his dream leaking into his voice. His older self merely chuckled. "Perhaps, it seems that your soul is different than mine, though just as disoriented and powerful. Blades, instead of Death it seems." he nodded sagely. Rin stared in obvious shock, "What do you mean? How did you become a legendary hero!" she demanded, only to be stopped by an ethereal, older version of herself.
The older her shook her head slightly. "I wouldn't even bother, this one offers his own soul to keep those who wish not to pass on in this world, heck, he was a Dead Apostle before I even knew him." Rin gaped.
Older Shirou chuckled, anyway, I would be happy to take part of the burden for Saber in return for shelter, I don't really want the grail or anything.
Random Sentence: People really love Life is a Game stories.