A few years ago, things weren't looking too chipper for Brendan Greene.

Having returned from five years living in Brazil, he was crashing with his parents in Ireland, collecting unemployment cheques.

"I was an unemployed photographer and designer," he says of that time. "At 35, there was not a lot of career options for me."

In Brazil, though, he'd started playing around with game design. His parents were a little sceptical, but he believed he had an idea that might just turn into something good.

The last 12 months have proved him right.

Brendan Greene, AKA Playerunknown, developed the idea for the game while living in Brazil. ( ABC News: Paul Donoughue )

A still-in-development version of the game he eventually created, Playerunknown's Battleground, was made available on PC via the platform Steam in March.

It's since become the most popular game of the year. It surpassed a million sales in less than a month. It then reached 4 million within three months — faster than Minecraft — and was past 13 million by the seven-month mark.

In September, it broke the record for the most concurrent players of any game on Steam, with more than 1.3 million people playing it at one time.

What's behind Battlegrounds' popularity?

The game is inspired by Battle Royale, a Japanese dystopian flick in which students fight to the death until only one remains.

In Battlegrounds, 100 players drop into a landscape from the air. The last man, or team, left standing wins.

To up the thrills, the playing field gradually shrinks, increasing the chance you'll run into another player and be forced into conflict.

In Playerunknown's Battlegrounds, 100 players fight to the death. ( Supplied )

Greene isn't certain what's made his game such a success, but says the freedom it presents players may be part of it.

"I loved the idea of these open worlds with no rules," he says.

"We provide a world or a playground to do battle, and how you do it is up to you."

Endless stories within a single game

The game lets you do all sorts of other-worldly things, but at its heart is something very human: survival.

"It's not against AI, it's not against computers, it's against other players, and there are 100 other players out there and everyone starts the same way," Greene says.

"You start with your clothes on your back and that's it. Everyone has the same chance to win."

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Chris Charla, the director ID@Xbox, which helps independent developers get their games onto the console, says the appeal of Battlegrounds is its versatility.

"It's so fun, when you are with people who have been playing the game, to tell stories of double kills, or the time you were just about to win and your motorcycle hit a rock," says Charla, who is in Australia for the PAX Aus gaming convention in Melbourne.

"The game enables all these amazing stories."

Greene is now thinking about what's next

The next title Greene creates will be survival-based, he says, but he has no expectations of another break-out hit.

And that's fine. Unemployment cheques are a thing of the past, after all.

"I don't think it's sunk in yet," he says of the events of the past year.

"In Brazil, I had to really worry about what I was spending my money on. And not having to worry about that anymore is nice.

"I have a 12-year-old daughter and I know that she does not need to worry ever — I can provide a great life for her. And that just makes me happy."