Flashback of what happened between disciple and whatever the sealed thing that said hi to her at the end of ch 19 is.
Seem to be something like
Disciple went mad/get corrupted by whatever made her think about destroying her 3 true souls.
Friend/sealed thing tried to save her from big oof by taking the last one and tried to hide.
Disciple killed friend for touching her balls.
My guess is that
Either disciple un-crazy herself or the friend absorb whatever made her crazy to un-crazy her.
Disciple went big depression and go reincarnate.
Friend seal herself here to avoid corruption making her do stupid things.