Heaven and hell. A realm of angels, a realm of demons.
Two opposite worlds connected by a "gate", a neutral vast stretch of space.
The two worlds rarely interact, due to the negative effects of
each realm's "silk" which cripples anyone from the opposite.
Queen Anastasia sent angel Sakura to hell to recover a forbidden mirror.
Sakura did her duty, but the influence of her mirror self "Kuroha"?
-- or perhaps some other cause? -- has rendered her almost powerless.
She can't return to heaven. She's trapped in the spirit world...