From what I can understand of the story, this part is a flashback or dream about Reimu's mother. The backdrop is black, and Reimu screams 'okaa-sama' when she disappears.
So, fixing tags to include 'hakurei miko'

Ya wasn't too sure until I got trough the basic tagging that is was a flashback.

Any tagging help in that regard is appreciated.

According to pixiv tag, this is Reimu though.

Title: 麗う夢02

rantuyetmai said:
According to pixiv tag, this is Reimu though.

Obviously, all the Hakurei miko are named Reimu.

Read on and you will find that it's Reimu that is performing the ritual.
Her mother is depicted as faceless.