CASE SOLVED!
Enough details have been uncovered and brought into connection to declare this case as solved. Please wait a bit for
post-game summary.Shou was flying over the lake, holding her Pagoda. Nearby, Rumia was heading towards Shou with intention to attack her. Shou did not notice the ball of darkness until it was close. When she noticed the orb, the attempted to defend against by shielding herself with her arm which was holding the Pagoda. Shou did not know Rumia was in the ball of darkness at all, she was caught off guard when Rumia directly attacked her. This attack startled Shou and caused Shou to drop her Pagoda. She then dove into the lake below to get the Pagoda back.
YESThe Byakuren crew has already settled into Gensokyo. So, Byakuren decided to start getting to know other large parties in Gensokyo.
She asked Shou to head towards the Scarlet Devil Mansion and have a chat with the occupants there. Shou brought her spear and pagoda just in case there was going to be danmaku battles.
She was flying over the Misty Lake when she heard a voice from below, right behind her : "IS THAT SO~". She turned her head to look for the source of the voice, instead, she saw a huge, black sphere heading towards her at high speed! If you had a volleyball shot towards you, you'd block it without much thought, right? And that's exactly what Shou did. Since Shou has never seen Rumia before, she had thought Rumia to be some kind of huge cannonball. She sure was surprised when her palms went right into the sphere like there was nothing.
She quickly realise it was just a sphere of darkness, so she tried to light things up with the light from her pagoda, but it wasn't in her hand! You see, when a person tries to block something, she opens her hands and push against it, right? But Shou was holding her spear and pagoda, and the items fell straight through the sphere with no one and no friction to hold onto them!
The pagoda was a extremely valuable treasure, so Shou dove straight into lake in search of her treasures.
And this concludes the first Touhou Black Story.
The details in your answer are a bit-off, but all main points are discovered.
I've had fun being the gamemaster, though the process did not went that smooth, I hope you all enjoyed this riddle I made.