Mikagura School Suite - Season 1 Episode 11 Nostalgic Triangle

After Otone runs off, Eruna becomes shocked when she notices some strange scars on Seisa's back. The next day, Otome apologises to Eruna, who asks her to help her train for the upcoming midterm battles. As Seisa stays away from the battles and shuts herself in, her Killing Art ability starts going out of control, causing temporary cracks to appear around the school. Investigating the school's photography club, Eruna stumbles into a different dimension and comes across Bimii. He explains that during her first year, Seisa was betrayed by the photography club upperclassman she admired, who seriously injured her with her ability, prompting Seisa to use her Killing Art to seal the clubroom away and shut herself in. With Seisa's Killing Art growing more unstable by the minute, the school itself starts becoming warped, with Eruna and Bimii falling through a sudden crack.