I do not own Code Geass or Touhou Project, they belong to their respective owners.
Prolouge: Mere Boredom
Cherry blossom trees that were almost everywhere in the land of Gensokyo had many of it's petals drift in the gentle breeze. Many average youkai, ones that are considered weak such as fairies, flew around in a humble and playful feat. Things were quite normal from what a resident of Gensokyo would say, with the fact that it's usually this peaceful and quiet most of the time. Sometimes an incident would happen and disrupt the peace, but it is always resolved by either Reimu, Marisa, or any of their acquaitances. Speaking of incidents, some are caused by greed, some by misunderstanding, but sometimes even boredom could even start an incident up.
In a traditional Japanese-style household, in the sea of cherry blossoms, a bored youkai sat at the porch of the house. This wasn't a weak youkai that's just asking to be taken out, but this wasn't just a strong one either. This youkai was one with a god like ability, one to manipulate any kind of boundary in existence. In fact, this youkai is also older than Gensokyo itself, and is quite knowledgeable on many kinds of things at the outside world. She knew the outside world had many alterations from her vast knowledge of all there is, but there was a few that she didn't have any knowledge of at all. Either way, some say that this youkai isn't responsible enough to have such an ability, and this youkai is known by the name Yukari Yakumo.
Why is she bored in the first place? Well, if you had the power to manipulate boundaries, know almost all there is, being one of the most dangerous youkai, what do you think she could do next? Who knows? Yukari had been boredly sitting at the porch of her house for quite awhile, wondering what would relieve herself of her boredom. Maybe I could peek a bit into the other places at the outside world to see what amusement it has to offer... Yukari thought as she sighed and opened a dimensional gap in front of herself to see what kind of place she got to watch for the time's being. First of all, this place looked quite advanced in technology, even more advanced than the usual outside world that most of Gensokyo knows about. Secondly, it seemed that there was some kind of battle with robotic machinery by how she saw no human being to be seen, but she felt it slightly. Thirdly, Yukari figured out after a bit that she's never seen this dimension before to recall.
She sealed up the gap, seeing that it didn't seem like the activity there would change. Then, Yukari was struck with an idea to relieve her boredom, to change the events of both Gensokyo and that dimension, and she knew what to do and how it would work. Getting up from her porch, now standing up, Yukari went through another gap that she had just opened, sending herself to get an aerial view of all of Gensokyo. She then began to think long and hard on who to choose from here to send into that dimension. Yes. That was her idea, to send 3 people from Gensokyo into that dimension, and she was very picky to choose who for this idea of hers. Finally deciding, Yukari opened up 3 gaps to take those determined 3 into the dimension.
There was one thing that Yukari had never thought about or even went over: why exactly there was a battle and what was the cause of it in the first place? If she ever found out, she wouldn't care at the slightest and let it go along. Returning back onto the porch of her house with boundary travel, Yukari walked back into her house to tell her shikigamis about her latest way to rid of her boredom. A slight smirk on her face was visible once she headed inside, and it was clear that she did start something that would be very disastrous.
A/N: So that's the prolouge for my CG/TH crossover. I can't really decide which 3 of the Touhou girls to send into the Code Geass universe, so I'm going to let you reviewers go and vote who. Also, with Touhou, it takes place after the events of Ten Desires, and with Code Geass, kinda before the battle in Narita. So review people!