Sprawling cities. Lush landscapes. Endless quests. World of Warcraft (WoW) is legendary for its intricate makeup. But what makes it the ultimate game changer is community. Sparking an era of large, social video games, WoW became one of the first globally popular titles—often spawning lifelong friendships, even marriages.

When Blizzard Entertainment launched the game in November 2004, it marked a turning point for massively multiplayer online games, or MMOGs. WoW wasn’t the first MMOG, but it took the format to a new level and mobilized a gaming phenomenon that helped spur today’s esports mania.

After a project to develop a new game fizzled in 1999, Blizzard pivoted that team to something more experimental, and two years later the company teased WoW at a European computer trade show, then kept iterating to fine-tune the game.

Within a decade of its release, WoW had accumulated 100 million unique player accounts in 244 countries and territories. One of the game’s signature features is the ability for players to join a group, known as a guild, where they can go on adventures to tackle various challenges, communicating simultaneously via voice chat and written text. Some challenges can take months to complete, and it’s not uncommon for people to spend thousands of hours playing together, building elaborate shared experiences.

The WoW team took steps to open a niche genre to a broader audience. Developers introduced relatively forgiving penalties (what hardcore gamers might call easier gameplay) to lower barriers to entry. They crafted compelling storytelling, with years-long arcs weaving from one expansion to the next and visual design that constantly sought to push the limits. Across seven major expansions, WoW introduced multiple state-of-the-art features, from daily quests—which gave players a guiding purpose to their actions and provided structure that had been lacking in other games—to a now-common matchmaking tool that automated the once-tedious process for finding other players interested in pursuing similar gameplay.

At its peak in 2010, WoW reached 12 million subscribers, and by 2017, it had become one of the top-grossing games of all time, with more than US$9 billion in total revenue. And the appeal endures: WoW’s 2018 expansion, Battle for Azeroth, sold a franchise-record 3.4 million units on the first day.