It was a career coup for Rajeev Misra. In 2017, the former banker, who had held a series of Wall Street jobs, was put in charge of one of the most formidable investing machines ever assembled.

His rise to the top of SoftBank Group Corp.’s $100 billion Vision Fund isn’t a traditional tale of corporate ladder-climbing. He succeeded, in part, by striking at two of his main rivals inside SoftBank with a dark-arts campaign of personal sabotage.

The tactics included planting negative news stories about them, concocting a shareholder campaign to pressure SoftBank to fire them and even attempting to lure one of them into a “honey trap” of sexual blackmail, according to people familiar with the matter and documents reviewed by The Wall Street Journal.

At stake for Mr. Misra was the opportunity to be the right hand of Japanese billionaire Masayoshi Son, SoftBank’s founder. He would help Mr. Son spread billions across the tech world, funding highflying startups such as Uber Technologies Inc. —and, recently, stumbling with a big stake in WeWork. The fund would expand SoftBank’s already huge footprint, which included a telecom empire, a microchip designer and robot makers.

Mr. Misra’s targets were Nikesh Arora, who was once heir apparent to Mr. Son as chief executive, and Alok Sama, a deputy to Mr. Arora who grew to work closely with Mr. Son on big deals. Mr. Arora left SoftBank in 2016; Mr. Sama left last April.