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.
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The transaction comes just as U.F.C. concluded its latest series of fights, perhaps the biggest in the organization's history. (The event, U.F.C. 200, drew more than 18,000 fans to Las Vegas but was marred by some controversy, including the absence of stars like Ronda Rousey, Conor McGregor and Jon Jones, the last of whom was removed after testing positive for an undisclosed substance.)
It will be a windfall for U.F.C.'s primary owners, the longtime casino entrepreneurs Frank and Lorenzo Fertitta, though they will stay on as minority investors.
The brothers bought U.F.C. in 2000 for just $2 million at a low point for the league, after the promoter spent years and millions of dollars battling to win approval from state athletic commissions.
Before the arrival of the Fertittas, U.F.C. had begun climbing in popularity, thanks to early stars like Royce Gracie and Ken Shamrock and a reputation for being just shy of civilized. But opposition loomed from Senator John McCain of Arizona, who derided the sport as "human cockfighting," and former Governor George E. Pataki of New York, who banned the sport from the state.
The early years under the brothers — who run the business through a company called Zuffa, the Italian word for "fight" — were still tough, with million-dollar losses weighing over the enterprise.
But the Fertittas took the brand to a new level, with more advertising, more effective social media marketing and better distribution through partnerships like the one with Fox. Licensing money for video games, clothes and more began to roll in. U.F.C.'s revenue was about $600 million last year.
Respectability came along with rules meant to curb the excesses of early fights and getting approval from each state's athletic commission. The last came this year, when New York lifted its ban on the sport.
And cross-promotional television programming like "The Ultimate Fighter" reality show elevated U.F.C.'s presence in pop culture, turning athletes like Chuck Liddell and Randy Couture into mainstream stars. Combatants began appearing on ESPN and in the pages of Sports Illustrated.
Successive generations of fighters, particularly Ms. Rousey, became even bigger celebrities.