World of Warcraft makes more money than the biggest Hollywood blockbusters every single month, has 11 million players and controls two thirds of the online gaming market, so when an expanded version was released, fans turn up in droves to be the first to get their hands on it.
WoW fans explain why they love it
World of Warcraft is more than just a game - it's a whole industry, selling everything from action figures to t-shirts, playing cards and even tickets to its own gaming festival.
Thousands of fans dressed in costume and queued overnight in central London to be the first to get their hands on the latest chapter in the saga, an update called Wrath of the Lich King.
"It's just a big social gaming experience," according to Paul Sams at Blizzard, the American company behind World of Warcraft.
"When we first started this back in the early 2000s, we thought it was going to be quite a niche type of game. We hoped to hit a million subscribers one day," he told Newsbeat.
"It's been far beyond anything we could have imagined. We all want to be heroes, but we can't in our everyday life.
"There were games before World of Warcraft but they were much more hardcore and that was quite intimidating for many people.
"We've worked really hard to try to make a really casual ramping of difficulty. So no matter what kind of thing you are in to, you are able to jump in and experience all of it."
World of Warcraft fans get into character for the midnight release
The features built into the new expansion pack might not sound like much to non-fans.
Players can access a new frozen continent called Northrend, level strength has been raised from the current max of 70 up to 80, new vehicles and siege engines appear for the first time, an achievement ranking makes it easier to track your progress and gamers can play as a new character called a Death Knight.
But for hardcore World of Warcraft players, these small changes are a huge deal.
"I'm really looking forward to exploring the Northrend area. It's the same graphics engine but they've done a good job of making it look different," 26-year-old gamer Lesley Harrison told Newsbeat.
Lesley has been playing online games together with her husband Mark since they were married six years ago.
'Easy to play'
"World of Warcraft is very polished and very accessible," she said.
"There were other games out before but they were so unforgiving and quirky. The interfaces were very hard to learn and there was a lot to get past before you could enjoy the game.
"With World of Warcraft you can sit down and on your first day you can get to some exciting content. That's pretty unique."
Lesley and Mark share a love of Warcraft and play together
Lesley and Mark have their computers set up side-by-side in the front room.
Mark told Newsbeat: "It would be really difficult if Lesley didn't play. It takes hours to do some things in World of Warcraft, so it's easy for the other person to get bored if they're not involved."
Lesley added: "We share different duties so if one of us is in the middle of something, the other one will go and make the tea. I don't think that would be appreciated if we didn't play together."
But with the average WoWer spending around 25 hours a week on the game, other couples are not so lucky.
'Destroyed by a game'
More than 3,000 people have joined an online support group called Warcraft Widows for relatives of addicted gamers.
"My wife started playing in 2007," according to one post. "She plays 12-18 hours a day and ignores all other life.
"She refuses to answer the phone. She refuses to return emails. I am destroyed by a video game and have no idea how to proceed."
That kind of dedication has helped make World of Warcraft one of the biggest money-spinners in gaming history.
It costs £9 a month subscription to keep playing the game online
Customers must buy the £24.99 disc to begin with and then pay a £9 a month subscription to keep playing online.
With 11 million registered users, it's not hard to see just how much money Blizzard is making every month.
It's also no surprise that rival online titles from Runescape to Lord of the Rings Online and Star Wars Galaxies have been fighting hard for some of that lucrative business.
But with a 60% market share, World of Warcraft already has a huge advantage over its rivals.
"There are a lot of people who want to go and play other games," said Lesley.
"They go but their friends just aren't there. They end up coming back to World of Warcraft even if the other game was perhaps better in their eyes. Friends are 90% of the fun. "
Hollywood has been eyeing the rise of online gaming suspiciously.
If millions of young gamers are spending 25 hours+ a week playing World of Warcraft then chances are they are not out at the multiplex watching the latest action movie.
But in a true case of 'if you can't beat them, join them', rumours are rife that a blockbuster World of Warcraft movie is on the way.
The film is said to be set a year before the events of the game and will focus on an all-new hero figure.
No director has been named so far but the budget has been set at $100m (£67m).
Or less than Blizzard earns from World of Warcraft subscriptions in a single month.
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