The Nikkei 225 index in Japan plunged by 12.4%, marking its worst day since the 1987 "Black Monday" crash. The index closed at 31,458.42, shedding 4,451.28 points. The sell-off was triggered by concerns about the U.S. economy and followed a 5.8% drop on Friday. Major companies like Toyota, Honda, and Mitsubishi UFJ Financial Group saw significant declines. The Bank of Japan's interest rate hike and a stronger yen have further pressured the market. Investors now focus on upcoming trade data from China and Taiwan, and central bank decisions from Australia and India.