>"Yes."
> "I cannot be bothered to monologue. Ran, if you please?" Yukari asks with a smile.
> "Gensokyo is not the only hidden realm from the outside world of Earth. This is one other, a much larger civilization. They are the Lunarians. They live on the Moon, and can hide their civilization from human eyes. Their technology would make the Kappa envious, and their magic is incredibly powerful. They see Earthlings as inferior, and detest us. We are like insects to them." Ran says.
> "Not too many years ago, shortly after Remillia and the Scarlet Mansion entered Gensokyo, Yukari decided to teach Youkai a lesson about getting over-ambitious, and, on the full moon, opened a portal to the moon, launching an... 'invasion' of sorts, which she knew would fail. This was to send a message. The Youkai who originated in Gensokyo, or ones who arrived recently and saw how many there were, were thinking of leaving, and attempting to make it in the outside world. This would be doomed to fail. The Lunar Invasion was to show the Youkai they could not set their sights outside Gensokyo."
> "Yukari had arranged this with the Lunarians, of course, they were prepared, and didn't take any lives. Because all that fought were two Lunarians. Against several of Gensokyo's most prominent characters. They took no damage whatsoever, no matter what everyone threw at them. Even a Master Spark fired from Marisa, who snuck along for the fun of it, was cut in two like it was a leaf." Ran says.
> "It is these two people arriving in Gensokyo that we fear the most. If they arrive personally to accomplish their mission, they will not hold back, and it is likely Gensokyo will be completely destroyed. The Lunarians see us as insects, and will treat us as such. These two are stronger than even Mistress Yukari." Ran finishes.
> "Thank you Ran. There is another history lesson as well, related to the aims of the Lunarians. However, feel free to ask any questions." Yukari says.
[Note: I've never actually read SSiB. I may have some details wrong.]