Five professional gamers are about to become the richest players in electronic sports history after winning a large chunk of tournament prize pool that's more than $10.5 million at the end of July.

While making a living off playing video games isn't unheard of, becoming a millionaire in the span of one video game tournament is certainly rare for the growing gaming and eSports industries.

You might expect that some of the money fueling the growth of the eSports industry in recent years — an industry whose audience surpassed 71 million viewers, doubling year after year, with a total of $25 million in prize pool money in 2013 alone — comes from corporate sponsors and organizations funding eSports ventures. But for the popular PC video game Dota 2's fourth annual international tournament, the $10.5 million prize pool was almost entirely raised by the community of fiercely dedicated fans who play the video game created by Valve.

These fans have been contributing to the prize pool for months leading to the tournament, aptly named The International 4, that will determine the best Dota 2 team in the world.

eSports tournament prize pools are not typically crowdfunded like Dota 2's International. Normally, mid-sized organizations put up a flat $50,000 prize pool. For larger companies like Riot Games and its League of Legends World Championships, a standard amount is given for the prize pool scales to compensate, usually hitting around $1 million.

For its international tournament, Valve tasked fans with increasing the levels of the tournament's prize pool with an in-game item sold via micro-transaction called the Compendium. Though Valve initially seeded $1.6 million to the prize pool for the International, the sale of the Compendium places the largest and most important tournament in Dota 2 in the hands of those loyal fans — a perfect example of Valve's general laissez-faire approach to creating and nurturing an eSports scene.

Unlike Riot Games and its construction of a professional league, Valve has largely stayed away from that same sort of direct construction of its eSports scene. Instead, there are a myriad of tournaments where players can complete and viewers can watch.

Normally, Dota 2 fans can buy in-game items from Valve's Steam Store, a popular gaming platform that's essentially Amazon for video games. In the months leading up to the International, Valve placed "the Compendium" in the Steam Store for sale.

For $10, a Dota 2 player could buy the Compendium, which offers a bunch of in-game exclusive, cosmetic rewards. The Compendium can be "leveled up" by either playing Dota 2 or continually buying more Compendiums. Every time you level up your Compendium, the rarity of your in-game rewards increases.

Valve took in 75% of the total proceeds; the other 25% went to the International 4's prize pool.

This is the second year that Valve has placed the Compendium in the Steam Store to increase the prize pool of the International. In 2013, Compendium sales increased the prize pool of the International to $2.8 million, and $1.4 million of that prize money went to the tournament's winners, Alliance.

The incredible increase over the course of one year is in part because Dota 2's player base has grown, but mostly because of Valve's decision to allow players to level up their Compendium, mostly through repeated micro-transactions.

"The player base has increased since the International 3 in 2013, but I think the loyal fans are really driving the Compendium sales forward — the ones that keep buying it over and over," Sam "Bulba" Sosale of the professional Dota 2 team Team Liquid told Mashable. "The repeated sales are way higher than the unique sales."

They're playing for $10 million. But what are they even playing?

Dota 2 originates from a modification of Blizzard Entertainment's 2002 Warcraft III: Reign of Chaos called Defense of the Ancients, or DotA — the game that gave rise to the Multiplayer Online Battle Arena (MOBA) genre. Though Dota 2 isn't owned by Blizzard, Valve kept the namesake.

Dota 2 is a more complex game than others in the MOBA genre like League of Legends. Even on the micro level, there are a lot more variables that a Dota 2 player has to be aware and in control of to be successful.

A screenshot of the Dota 2 map shows the two opposing sides, the three lanes and the position of the main structures — the Ancient — in each sides' base.

The Dota 2 map is split between two sides: the Radiant and the Dire. Maps are divided by three lanes defended by three autonomous towers in each lane. Players try to barrel down these lanes to get to the enemy team's Ancient — the main "base camp" structure in Dota 2. If a team is able to destroy the enemy's Ancient, they win the game.

Players control one of more than 100 different heroes all with different attributes and roles during the fast-paced game. These roles are highly varied and many combinations of roles could work in a successful team composition.

With $10 million at stake, how has the prize pool affected the competition?

The competition in Dota 2 is fierce. Any of the top-tier teams can take a game off of the others at any given day. Now consider that $10 million is on the line — an amount that will surely change the lives of these players — and the amount of teams hungry and able to compete for the pot is sure to rise.

See also: How a YouTube Documentary Gave New Life to a Nintendo Classic

Specifically, for this year's International, Sosale identified two teams in particular that are certain to be huge threats in the tournament due to their unique team play styles: China's DK and North America's Evil Geniuses (EG for short).

Realistically, there are eight or nine teams that can win the tournament, new and old teams alike. With the prize pool ever increasing, that number of teams who can potentially vie for the first place prize also increases due to constant training.

Team Alliance holds the Aegis of Champions after beating Natus Vincere in the Grand Finals of the International 3 on Aug. 11, 2013 in Seattle, Wash. Image: Dota 2

"As a player, the money is great and all, but I want to do well just because I love the game," said Sosale. "The players that will do the best at the International are the ones that aren't concerned with the money. You have to want to win because you love the game."

You can watch the best Dota 2 teams fight for the title ,- and ultimately a large chunk of the $10.5 million prize pool — at the Key Arena in Seattle via Twitch live stream from the round robin stage starting on July 8 to the grand finals on July 21. If you're unfamiliar with Dota 2 or the MOBA genre, there is a newcomer-friendly stream as well.