Anime with a "everything goes to hell all of a sudden" arc

A major arc where either the cast is forced apart, or inserted into a deadly scenario with dreadfully low hopes of survival. They're taken from the comfort of their "usual" and put into a wholly new environment that has the reader/watcher freaking out along with the characters.

Preferably happens not immediately, like for example in Akatsuki no Yona or similar anime where the premise is built on such an event. Rather, it's built up over time.

Any good anime/manga with such an arc? Examples would be JJK's Shibuya Incident, Danmachi's 4th season, or for those playing the game, Fate/Grand Order's Lostbelt arc.