The ancient Persian god that may be at the heart of Game of Thrones

The sixth season of HBO's hit series Game of Thrones, starting today on Sky, will likely bring into focus an important historical element from the fictive universe of George R R Martin's novels, upon which the television show is largely based: the legend of Azor Ahai.

(To be clear, we are as much in the dark as anyone else about what will unfold in the medieval fantasy show's new season and have no desire to divulge any spoilers. The following is based purely on knowledge of the previous seasons and Martin's books.)

Azor Ahai is a mythological figure in the books and the show, a demigod warrior from an ancient time who triumphed over darkness in a great battle while bearing a magic weapon blessed by R'hllor, a deity also known as the Lord of Light or the fire god. There's a prophecy that he will be reborn at a time of similar peril and that's where the speculation for the upcoming season comes in.

"There will come a day after a long summer when the stars bleed and the cold breath of darkness falls heavy on the world. In this dread hour a warrior shall draw from the fire a burning sword," intones one of Martin's characters in the second book of the series.

"And that sword shall be Lightbringer, the Red Sword of Heroes, and he who clasps it shall be Azor Ahai come again, and the darkness shall flee before him."

The story of the messianic hero is, of course, a universal one. It's at the heart of numerous religious traditions and stirs the human imagination like few other tales.

But in the feverish world of Game of Thrones fan-sites and discussion boards, a fair amount of thought has been given to the possible historical provenance of the Azor Ahai legend.

Martin himself has said he developed the idea of the monotheistic, fire-worshiping faith surrounding R'llhor from Zoroastrianism, a pre-Islamic religion that emerged thousands of years ago in what's now Iran.

Beyond the purifying importance of fire in rituals in both faiths, Iran's ancient traditions gave realms further west the first understanding of a moral universe shaped by a binary good and evil.

God in Zoroastrianism is known as Ahura Mazda, an omnipotent, supreme figure who created the twin spirits of good and evil - Spenta Mainyu and Angra Mainyu, also known as Ahriman.

There are interpretations of the tradition that associate Ahura Mazda purely with the former, a life-giving creator, and place him at odds with Ahriman, a being that ultimately must be defeated.

Here's more from the entry on the deity in the Encyclopedia Britannica:

"To aid him in attacking the light ... Ahriman created a horde of demons embodying envy and similar qualities. Despite the chaos and suffering effected in the world by his onslaught, believers expect Ahriman to be defeated in the end of time by Ahura Mazdā. Confined to their own realm, his demons will devour each other, and his own existence will be quenched."

This sense of eternal struggle and combat with an evil Other is hardly unique to Zoroastrianism.

But it is the most clear blueprint for the cosmic clash being set up in Game of Thrones, where Ahura Mazda is a composite of both R'llhor and Azor Ahai, and where a righteous hero of fire will eventually take on a relentless evil from the lands of ice.