a sudden and dramatic turn; a situation changing abruptly and decisively (yojijukugo for a precipitous change).
会議での議論は一つの発言をきっかけに急転直下した。
The discussion at the meeting took a sudden turn after one remark.
天気が急転直下して嵐になった。
The weather changed abruptly and turned into a storm.
物語の展開が急転直下し、読者は驚いた。
The plot took a sudden turn, and the readers were surprised.