The acquisition of Sprint Corp. was supposed to help Masayoshi Son realize his vision of transforming SoftBank Group Corp. into the world¡¯s most valuable company. Instead, the 2013 deal has become his biggest setback so far, dragging down SoftBank shares and cutting into the billionaire¡¯s wealth.

SoftBank dropped 7.9 percent in Tokyo on Monday to ?5,111, falling to its lowest level since it closed the Sprint deal 2? years ago. Son¡¯s fortune has shrunk by $3.2 billion over the past 12 months, according to the Bloomberg Billionaires Index, as the Japanese company¡¯s stock plunged.

The Sprint deal has proven a dramatic blow for Son, one of the nation¡¯s most enigmatic entrepreneurs who has said he has a 300-year plan for SoftBank and wants to build an Internet empire unrivaled in ¡°profit, cash flow and market value.¡± Instead, as Sprint losses mounted, Son found himself spending hours a night on the phone with its engineers and executives. The prospects of a turnaround may be slipping away, taking away precious time from the 58-year-old as he tries to secure his legacy.