There's a Subtle High Deflation in the World. Play Games without Accumulation - S1.E10

This episode, which is anime-only, takes place during the same time as episode eight and shows some other strange things that were going on in the background. A demon living inside an old video game is awoken and takes three of the student council members from Nagi's school inside the game where they must defeat her to return to the real world. Hinagiku, the student council president, arrives soon after and they go to destroy the demon, but she ends up ending it herself.