It was the stuff of gamer dreams: Brandon Beck and Marc Merrill took a passion for World of Warcraft-style online games, turned it into League of Legends, and watched their creation become the most popular PC game in the world. By 2016--the year Inc. named the duo's business, Riot Games, as its Company of the Year--League of Legends (or LoL, for short) counted over a hundred million players every month and generated more than a billion dollars in annual revenue. Fans of its global professional league packed arenas like Madison Square Garden in New York City. Beck and Merrill themselves seemed awed by their creation. "Imagine we invented basketball," Merrill told Inc. last year.

A year later, Beck and Merrill now face their greatest challenge since starting the company: finding a new hit. On October 16, Beck and Merrill announced that they were handing off day-to-day operations of the company in order to work full time on building Riot's next big game--an opus that has been promised for years yet never released. Until now, the plural name of the company, Riot Games, is a sort of misnomer--LoL has been its only major franchise. Other than a handful of casual mini games and a tabletop board game, the perfectionist founders haven't developed anything they like enough to put out in public.

The move comes as the popularity of LoL is leveling off; the game is attracting fewer and fewer "noobs," the company's name for new players, who must spend hours learning the game's complexities. Eight years since LoL's original release, Merrill and Beck must build a game or games that will carry the company for the decade to come.

"The [League of Legends] franchise is maturing, which is a polite way of saying it's getting old," says Joost Van Dreunen, chief executive of SuperData, a research firm that tracks the games industry. "When you look at the games industry, you have a lot of one-hit wonders, just like with musicians.... Just because you have accomplished success in the past is no guarantee for success in the future. It's a big question mark."

It's hard to overstate the odds-defying success of LoL, both as a game and as a business. The game, like any great team sport, rewards both individual skill and collective ingenuity. The object is simple: Two five-player squads face off to destroy the other team's base on the opposite side of the map. Myriad complexities keep the gameplay exciting: Players must work together using special skills and unique characters known as "champions." They must battle monsters in the jungle between the bases to strengthen their side. Ambushes change the momentum of the game; teamwork overcomes raw skill; underdogs sometimes capture breathtaking victories. For the game's hundreds of millions of players, it's an addictive mix. Many devote dozens of hours a week to playing the game, battling to rank among the game's best.

LoL, the business, pulls off no small feat either. The game is free to download and play, yet the company pours hundreds of millions of dollars into its development--improving its graphics, developing new characters, covering the massive bandwidth cost. The bet is that a high-quality free game will attract a broad base of players, and that a small segment of LoL players will be so passionate--and spend so much time in the game--that they purchase hundreds of dollars' worth of items to customize and decorate their experience. This has worked: In aggregate, these purchases generated some $1.9 billion in sales in 2016, according to SuperData. So far this year, LoL has sold another $1.5 billion worth of virtual stuff.

As the popularity of LoL plateaus, Merrill and Beck must create a game that succeeds as spectacularly as their first work. Yet now they play on a higher difficulty level: The free-to-play model they pioneered is no longer novel. Competitors abound, vying for the same hardcore PC gamers, and their fans' expectations are high. And so far, Beck and Merrill have been reluctant to bet the company's future on any new creation.

Both men have a perfectionist streak -- it's one of the reasons why League was so successful for so many years, after all. Among LoL fans, a new Riot game has been rumored for years, and, according to people within the company, they have been at work on various approaches. In 2013, an unreleased prototype of a sort of digital card game leaked, but was never officially launched. In 2015, some employees' Twitter bios indicated that they were working on a secret new project, although they never explained what it was.