Paul Singer’s Elliott Management is the world’s biggest, most successful activist hedge fund, according to a recent Fortune article that reveals what it calls  the “sophisticated—and often controversial—tactics” that have given them that distinction.

Activists, the article explains, use ownership stakes in public companies to “pressure them to change in order to boost returns—whether by restructuring their businesses, shaking up management, or even putting themselves up for sale.” In the past five years, it says, Elliott has launched campaigns at more than 50 companies in at least 12 countries, with only one of those ending unsuccessfully.

But Elliott’s tactics have been called into question by many. “Through interviews with more than 40 people who have dealt with the hedge fund,” the article reports, Fortune has gathered “previously unreported details that reveal just how far Elliott will go to win.” These details include a variety of untoward allegations regarding the use of private investigators and other “dirt-digging” tactics, as well as specific examples of deals the hedge fund has facilitated over the year and the players involved in them.