I watched Ju-on Black Ghost and didn't understand anything, but I'll try to explain.
The protagonist (Tetsuya, the first to die) heard knocking on his bedroom door, coming from his apartment.
At the end of the second arc, we see that the blows came from the spirit, which was banging the young man's head against the wall.
Yuko (Testsuya's neighbor) and this Yuko sees the black spirit killing Testuya and she also heard the noise. The curse connects two timelines to each other.
(Testsuya and Yuko)
Fukie was the girl under Yuko's (the nurse's) care. It's worth noting that Fukie had a twin sister whom she absorbed, and this sister is the source of the curse.
Exorcismo: a marika(tia da fukie) fez um exorcismo, mas a maldição enganou e a maldição agora controlava fukie
Kiwako (Fukie's mother) sees Fukie's spirit, the White spirit, which contrasts with the Black ghost.
Chronological order of deaths: Fukie Miako Kiwako, Ayano Toyota, Testuya e Yuko
(The curse begins from the end to the beginning)