falling headlong; plummeting; sudden crash
A dramatic, literary expression for a swift, headlong fall from a great height, often used figuratively for a sudden collapse or ruin.
His life took a headlong plunge into ruin.
Stock prices plummeted with the force of a sudden crash.
急転直下 means a sudden turn of events leading to a swift resolution, while 九天直下 specifically emphasizes a dramatic fall or crash.
A yojijukugo (four-character compound) combining 九天 (the highest heavens) and 直下 (directly below), literally 'falling straight down from the highest heavens'. The exact historical derivation is uncertain, but it is used to describe a precipitous fall.