In order to save Satoko from her uncle, Shion, Keiichi and Rena decide to kill Teppei. But after the deed is done, Rena starts to act strangely, and Keiichi thinks he hears Oyashiro-sama talking to him. Mion notices the change in her friends behavior and takes Shion’s place to find out why they are acting this way. Later on, Satoko develops the Hinamizawa syndrome, like in Tatarigoroshi-hen, and kills Shion before killing herself.
Woah, woah, woah.. Satoko? Killing people?
This sounds like an awesome arc.