SAN FRANCISCO — Ultimate Fighting Championship seemed for years to be a curio, a niche sport for those interested in seeing combatants pummel one another in an octagonal cage.

But U.F.C. has become a global sports empire — and now commands an eye-popping price tag.

The league, which promotes mixed martial arts, is expected to announce as soon as Monday that it has sold itself to a group led by the talent giant WME-IMG for about $4 billion, according to people with direct knowledge of the matter. Backing the deal are the private equity heavyweights Silver Lake, Kohlberg Kravis Roberts, and the investment firm of the billionaire Michael S. Dell.

The deal highlights the power and reach of the 23-year-old U.F.C., whose fights are now shown in more than 156 countries and take place in all 50 states, and which claims millennials as some 45 percent of its audience.

For its new owners, the league represents a prime source of content, particularly in the digital arena. Beyond its headline fights, which the company asserts are the best-selling events on pay-per-view TV, U.F.C. over all generates roughly 2,000 hours’ worth of material each year, much of it available on its Fight Pass streaming service.