Glorious summer reigns in Paris, but inside the Porte de Versailles Exposition Centre it's dark, cool and a little gloomy.

Row upon row of glowing screens are the subject of intense attention from huge, predominantly youthful crowds. Enormous posters and banners are adorned with artwork of elves, dwarves, orcs and other fantastical races, while lengthy queues snake their way around merchandise stalls where trading cards, statuettes and posters are displayed behind glass like museum artefacts.

For one weekend only, this southern district of Paris has become the capital of a sprawling, multitudinous nation. Amid the thousands who teem to the grandly titled 2008 Blizzard Entertainment Worldwide Invitational are people from 50 countries. Conversations in dozens of languages buzz around as you pass through the halls.

They are united by one thing - World of Warcraft. Created by the Californian game developer Blizzard Entertainment, WoW is a sprawling online fantasy realm in which players can adopt the persona of a hero and fight alongside thousands of others against an assortment of monsters, villains, dungeons and quests.

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When WoW was launched three years ago, the world's most successful comparable online game had nearly half a million paying subscribers, a figure widely considered to be the largest audience any such product could enjoy. Today, World of Warcraft is anticipating signing up its 11 millionth customer. Enjoying vast audiences in North America, Europe and Asia-Pacific regions, it is one of the most commercially successful pieces of entertainment in history.

For the people who have come to Paris for the Worldwide Invitational, the impact of World of Warcraft is far greater than any statistics can define. The game has become a part of their cultural landscape, as much a touchstone for their vocabulary, storytelling and humour as influential books, albums and films have been in previous decades.

At its simplest level, this expresses itself in the vast number of in-jokes that float around the convention centre. These range from T-shirts with awful puns on phrases used in the game to a dance competition in which participants try to replicate the body-popping moves of their characters. The popular comedy show South Park has based an episode entirely around WoW jokes.

Vastly more dedication to World of Warcraft's culture is displayed by those who turn up to the event in costume. Cosplayers, as they're known in the varied subcultures they inhabit, are an easy group to deride. Speaking to those who have created these costumes, however, reveals a passion for their creative, artistic hobby which is hard not to admire.

Mike Morhaime is co-founder of Blizzard Entertainment. “One new thing that we've had to do is to protect players from each other,” he says. “People don't always get along - we have to enforce behavioural rules. It's part of our job to look out for the players and ensure that we've got an environment they can have fun in.”

Speaking about the game's community, Morhaime's eyes brighten. “You get players who meet in the game and end up getting married... I actually get wedding invites sometimes. Then there's the costume stuff. That makes me realise how strong our community is. I'm blown away by how creative the players are in recreating these characters.”

Lisa Young, 26, from London, has travelled to Paris for the event, bringing with her an extraordinary costume based on the attire of her own in-game character. With flowing robes and intricately modelled armour pieces, the outfit is stunning. It represents, she tells me, about three months of work. Like the many other costumes on display, it demonstrates creativity, skill and patience - driven and inspired by enthusiasts' love for the fantasy world that they have embraced.

Arguably the greatest success of World of Warcraft lies in its ability to become a major part of the social lives of millions. Spending several hours a week (in many cases, several hours a day) in the game's fantasy world is something which, most of the players here tell me, they couldn't conceive of doing without their friends in the game.

Organised into teams called guilds, WoW's players help and support each other's progress through the game, and in the process chat to one another, discover each other's histories and forge firm friendships. For some, this event is an extraordinary opportunity finally to meet face to face people with whom they have adventured for countless hours.

Anecdotes about the firm friendships established across great distances flow freely from the players I talk to. Bernadette McKeegan, who has travelled to the event from Wales, plays in a guild largely made up of Dutch players, she tells me - but many of them made the trip to Britain for the wedding of a fellow player, signing a large card for the happy couple with their in-game names.

Even stories of family reconciliation crop up. A recently divorced man in his forties tells me that he has reconnected with his teenage son, who lives with his former wife, by playing the game with him in the evenings from opposite ends of the country. They now play together in the same guild, and have come to Paris together for the event.