What was it like when the cainhurst rulers were first introduced to the katanas/Japanese swords?
This is a very good question my friend ;)
I assume that at some point Cainhurst stroke a commercial deal with the Easter Lands (the same from which Yamamura is from) or perhaps even conquered some of their territories during a previous war and claimed the secret of their blacksmithing arts as a war trophy. Even the Royal Guards’ silver-thin armor looks more modern and exotic than those wear by the statues in Annalise’s chamber or the full plated ones we can find in the chalice dungeons.
Certainly, the use of eastern weapons is not something that they inherited from the Pthumerians!
I guess the main reason why the nobles wanted to change their arsenal was the looming threat of the Plague of the Beasts. Heavy armor and big swords and lances are fine as long as you fight other humans but Cainhursters were dwelling in the chalice dungeons possibly even before Byrgenwerth did and thus they had to adapt their combat style just like Gehrman would do later on.
Another good question would be “was the Chikage created as a blood blade from the start or it was because the Vilebloods used it that its nature got corrupted?” I haven’t tested this, but I’ve heard that Yamamura never transforms it when summoned as a phantom. Can someone confirm this? That would be really interesting if true.
— And to add some silliness to this post: here’s what Maria has to say about this in my fanfic: [Warning: light Gehrmaria & mention of a shirtless Hidetaka Miyazaki. You have been warned. I swear, this is a serious fanfic y’all]