When it began it was just a computer game. Now it is seen as a cultural force that sparks love affairs, breaks marriages and creates “sweat shops” to satisfy a black market in virtual goods.

World of Warcraft marks its fifth birthday today as something more than just an online role-playing game where users become wizards, warriors, orcs and elfs.

“It has had an enormous cultural impact,” said Tom Chatfield, author of Fun. Inc, a book about the growth of the games industry. “It has proved that online gaming can make huge profits, making a billion in revenue a year. It has proved that gaming could be for a truly global audience.”

Analysts say that its popularity has paved the way for other blockbuster games. This month Call of Duty: Modern Warfare 2, made a record-breaking $500 million (£303 million) within 24 hours of its release.

Others argue that the game’s success has suffocated rivals. “Pretty simply, it’s one of the best games ever made,” said Oli Welsh, a writer at Eurogamer.net, the video game news website. “But it’s a mixed blessing for the games industry. People are trying and failing to match Warcraft, and get enough players to take it on. Other games have been strangled by it.”

An estimated 12 million people play the game, paying a monthly subscription of about £9. They spend many hours taking on quests and doing battle with each another, building up their character — or “levelling up” as it is known in the game — to gain powers and abilities.

But it has a dark side. A thriving black market for virtual goods to use in the game, such as swords and animals, has emerged. Some top-level goods take thousands of hours to earn, and impatient players are willing to pay up to $5,000 on eBay to get them.

The real financial rewards on offer have created an underground industry, where players play in slave-like conditions to obtain the items and sell them on. “You effectively have sweat shops in China,” Mr Chatfield said. “If you were to visit one of these gold-farming things, you’d find a building with maybe 20-40 people in it, playing in shifts on banks of computers. They do it because they can earn more money doing that than producing garments in factories.”

The addictiveness of the game has been accused of destroying relationships, creating “Warcraft widows” as players set it above anything else.

One “widow” told The Times: “We’ve had to enter into some heavy negotiation to work out the nights and weekends when he can play without me nagging to stop. Because it’s not just the odd half-hour. Sometimes if he’s locked into a group he’ll break only to eat and visit the loo.”

Blizzard Entertainment, which owns the game, will not reveal the average time that players spend on it, but academic research estimates that it is between 25 and 40 hours a week.

This obsessiveness has even led to violent consequences. Last week, police in France said that they had thwarted a plot by a boy, 13, to murder his school teachers. He was allegedly addicted to Warcraft. He was found by officers with his father’s shotgun and cartridges.

In 2007, two parents were arrested for allegedly killing their child after becoming obsessed with the game.

However, fans say that the game’s popularity stems from the social atmosphere that it has created. In the game, people often have to join together to compete quests. Mr Chatfield said: “I know of several people who have got married through meeting in games. I even play the game with my wife.”

Blizzard said that they would mark today’s anniversary by introducing surprise elements into the game, such as the “Onyxia Brood Whelpling Pet” as well as new “bosses” for players to battle against.

Fans are also awaiting the latest “expansion”, which is an update to the game. Cataclysm is likely to be the biggest release of next year.

Mike Morhaime, the president and co-founder of Blizzard, said: “We never expected the series would grow into a social and pop-culture phenomenon. We’ve always simply made the games that we wanted to play ourselves and I hope that approach will continue to serve us well in the years to come.”

The darker side

Attempted murder In October 2008, ABC News in Australia reported that Zhenghao Shen, 21, a student, had stabbed a friend in the head with a knife and nearly severed one of his fingers. The reason for the attack? The victim had asked Shen to turn down the volume on his World of Warcraft game. When Shen refused to compromise, the victim challenged him to a fight. Shen responded, turning from his computer screen with a blade

Divorce A Californian woman, 28, who identified herself as “Jocelyn” told Yahoo Games that she divorced her husband of six years after he spent all his free time on World of Warcraft. Jocelyn ended her relationship with her husband, Peter, by explaining to him: “I’m real, and you’re giving me up for a fantasy land. You’re destroying your life, your six-year marriage, and you’re giving it up for something that isn’t even real”

Suicide The parents of a Chinese boy, 13, who died after jumping from a building, attempted to sue Blizzard Entertainment, the developers of World of Warcraft, because they believed that the game led to the death of their son. The boy’s parents said that he had jumped on the morning of December 27, 2004, after playing World of Warcaft for 36 hours without a break in a “game hall”. Zhang Chunliang, an attorney and a well-known campaigner against the game in China, supported the parents in their claim