Media playback is unsupported on your device Media caption Japanese artist Hajime Isayama: ''I had a dream of no limits''

Man-eating giants called Titans are stalking the Earth, and what remains of humankind is cowering behind vast walls in a dystopian future.

The dark vision of the Attack on Titan series has made its reclusive Japanese manga artist Hajime Isayama a global best-seller and cult figure.

His books have sold more than 52 million volumes worldwide and been translated into multiple languages including English, Mandarin and German. It went on to inspire an equally popular anime and now a two-part live-action film adaption.

Image copyright Hajime Isayama, Kodansha Image caption The original line drawings of the graphic novels have been turned into animation and live action films

In an exclusive interview with the BBC, Isayama has revealed that he nearly gave up his dream after struggling to find a publisher.

To his fans, there is more to the series than commercial success. Here are a few reasons why people love Attack on Titan.

Its unconventional creator

Isayama came up with the disturbing premise after playing a computer game which saw alien invaders attacking the world. "I thought it would be interesting if those monsters ate humans," he told the BBC.

He has been subjected to death threats posted online, but to his fans he remains a beacon of inspiration and creativity who struggled through rejection until he found success.

"Isayama is one of my favourite authors because he had an idea so strange but yet he did not give up," said a fan who commented on a popular Reddit thread about the series.

The heroes don't always win

Image caption Lead character Eren often finds himself torn between his past and his goals of wiping out Titans

Others appreciate the sheer epic scale of the story. With every victory comes strife and turmoil for the heroes.

Lead character Eren finds himself caught in a struggle after seeing his mother eaten by a Titan which destroys his hometown.

His journey to defeat the Titans soon reveals truths about them and other characters.

Image caption Fans have praised the series for its frequent plot twists

"This makes it feel real to me. Isayama's characters suffer a lot of problems and setbacks and have to realistically work around that," said one Japanese fan on Reddit.

"'Attack on Titan' is no typical Shōnen manga where good guys rarely die... In it, the villains more often than not win and every rare victory for the protagonists will often cost them dearly, so that they can't even experience the joy of celebration, that's what appeals to me," another fan said.

You can also see this story in Japanese on our new Japanese language website

この記事の日本語訳は、BBC日本語サイトでもお読み頂けます。

A gloomy grey world

Image caption The giant Titans lack reproductive organs and get their energy from sunlight

Image caption Gore and violence are common themes in the series, where man-eating Titans devour what's left of humanity

Some enjoy the universal themes of "hopelessness and loss".

"There is a serious and dark tone to the series, with a desperate struggle to it that makes it refreshing," said one fan, who added that the fact that it is not entrenched in Japanese culture makes it accessible to those unfamiliar with manga.

Many say the mystery, adventure and the focus on the secrets of the communities in the series have lessons for all students of the human condition.

Film footage courtesy of Hajime Isayama, Kodansha/Attack on Titan Production Committee