It stands to reason that they used a similar method to how Tachibana built her powers. Yuri investigates the computer lab.
It turns out someone has been stealing computers. And there's a suspicious underground passageway leading from the lab to the ruins of the Guild.
Many people choose to move on, leaving only a handful to fight the shadows.
Yuri is engulfed, and finds herself in a classroom, in a classic anime scene. But she doesn't play along and rejects the fantasy.