"They're all so incredible. They managed to speak the latest lingo as soon as they came back to life. There's nothing wrong with their grammar, vocabulary, or pronunciation. Oh, but it seems I can speak the modern tongue, too. There must be some kind of hidden rule."
Byakuren Hijiri is my older sister. She's a great magician.
I can't help but sympathize with you. Unfortunately, your journey will not get any easier.
Buddha and Youkai
It's said the first youkai were gods that fell from grace long ago. However, it's much more likely that the first youkai were manifestations of primeval human fears―specifically, the fear of Mother Nature's incomprehensible laws and dangers. The teachings of Buddha came later. The people in charge wished to instill the belief that Buddha is much stronger than anything that could threaten humans. In this land of myriad gods, the masses needed to believe that all beings seek the salvation of Buddha. There is nothing more powerful than mercy.
Anyway, this is what led to so many youkai being named after Buddhist priests, like mikoshi-nyuudou (lit. see over priest), umibouzu (lit. sea priest), and hitotsume-kozou (lit. one-eyed priest-in-training). Surely, they all want to be saved.
Flying Treasure
There seem to be a few discrepancies between the history of Gensokyo and that of the outside world. But there's nothing strange about that. History varies along with time, location, and culture. Apparently, Saint Myouren, a virtuous priest, existed in the outside world. He possessed the Flying Storehouse that later transformed into the Palanquin Ship, along with the many treasures rumored to fill its hull. It's possible that Byakuren was the one who turned these things into magic items.
The Palanquin Ship
The legendary Flying Storehouse transformed into a ship that can soar through the sky. It appeared in the skies above Gensokyo on its way to pick up Byakuren, who was sealed away in Makai as punishment for being a youkai monk. Palanquins are transports traditionally used to carry royalty and nobles, so the name of this ship indicates how much Byakuren's youkai disciples respect and adore her. The ship must be imbued with an incredible amount of magical power to be able to travel between Gensokyo and Makai.
Myouren Temple
Byakuren, a Buddhist priest, resorted to magic to stave off death. Many centuries later, she founded a temple and named it Myouren after her brother. However, this temple was also the greatest treasure her brother left for her―the ship her disciples used to bring her to Gensokyo. What human weakness compelled her to use magic? Was it her grief from losing her brother, or could she simply not overcome her base desire to preserve her own life? Perhaps Myouren knows, but since he has already transcended death, they will only see each other again at the very end.