(repost from a few months ago, correcting bits I misremembered)
It was on youtube a few years ago maybe 2018-2019 when I saw it. I wasn't into vocaloid then but I remember It having not many views, under 1-2k? The video was old, from 2009-11 era and had the title in japanese.
The actual song had a strange eerie vibe, I think it was Luka or Gumi singing.
It was slow at the start with wind noises, and the pv showed this ocean before cutting to a cage with a girl in it. she had dark hair, maybe purple or black. the light was all cold. it zoomed on her eyes and her eyes turned into a stormy sky and the beat dropped with heavy piano and got faster. there was a very specific bit where the girl was looking in the mirror somewhere, and crying, then she stabbed her reflection and the mirror shattered and the shards turned into this peaceful looking other girl with pale hair and a kimono/long dress, with petals flying around
I'm not sure, but I think she had 4-6 white, feather wings...? I don't remember what came next, except a LOT of flashy frames. near the end was a part where the pale hair girl flew down from the sky and kissed the Dark hair girl, before turning into petals or feathers. The screen went black, and the Dark hair girl was in her cage again, with a music box in front of her. as the screen faded into black, the music got slower like a music box winding down.
I'm pretty sure it was translated, with pink-red subtitles