With “Game of Thrones” about to start its penultimate series, and the showrunners at HBO already muttering darkly about five TV spin-offs from its vast fantasy narrative, other networks are eyeing the approaching gap in the market with interest. Last week Netflix announced that it is starting production on “The Witcher”, adapted from a string of books by Andrzej Sapkowski, a Polish writer, that has also inspired a series of immensely popular video games. Like “Thrones”, “The Witcher” takes place in a grubby and blood-soaked medieval fantasy world; like “Thrones”, it conjures a labyrinth of competing political allegiances and cynical personal loyalties; and like “Thrones”, the characters all swear like troopers, swordfight all day long and enjoy slinking about by firelight with their tops off. There are dragons, too.

It’s easy to see what might appeal about “The Witcher” in an entertainment landscape accustomed to George R.R. Martin’s brand of grim fantasy, although Sapkowski fans might tell you that their chap got there first. Formerly a translator and travelling fur salesman, he published his first “Witcher” story in 1986, a decade before the first book in Martin’s series “A Song of Ice and Fire” appeared. It introduced a white-haired, yellow-eyed mercenary called Geralt of Rivia, who travels through a miserable fantasy-Renaissance landscape getting into perilous scrapes and slicing up monsters for money. Geralt is a witcher, the term that Sapkowski uses for a squad of deliberately mutated humans who use cocktails of poisonous drugs to boost their reflexes and magical powers, and he is the first of many anti-heroes in a series whose morality status is permanently set to “it’s complicated”. Geralt’s pals include a ludicrously vain harp-strumming poet, a clan of ancient sorceresses who use magic to keep themselves looking 25 and beautiful, various spies, bandits and former soldiers, and an ashen-haired, sword-swinging princess called Ciri, around whom many of the later books revolve.

The “Witcher” stories have been genre hits in Poland, but they can read oddly in English, in which a combination of Sapkowski’s dialogue-heavy writing style and some slightly alien translation work produces unusual results. The books paint a striking picture of an amoral world, shot through with racism, sexual oppression and generally bad vibes: many of the early stories present skewed and bloody twists on Slavic myths and fairy tales, while the later books’ vision of society is scarcely more encouraging. Sapkowski’s world is perpetually at war: the humans repress the elves, who band into terrorist squads to fight back from the shadows; Geralt and his witchy friends are shunned as unnatural monsters by most of the people they meet; and there’s a gruesome long-running plotline about who gets to profit from Ciri’s “royal blood” by impregnating her. But it can often be hard to observe the intricacies of these relationships through the forest of wooden dialogue. I have just opened a copy of one of the novels to a chat between Geralt and his lover Yennefer. “We – and I mean witchers and servile golems – always act sensibly,” he observes. “Since the limits within which we operate are clearly and explicitly demarcated.” “Well I never,” she responds. “You’re taking umbrage like a tart whose lack of chastity has been pointed out to her.” At moments like this, either the author or the Babel fish is malfunctioning.

Dragon? Tick. The similarities to “Game of Thrones” are striking

Whether for this reason or another, most English-speaking audiences will have first encountered Sapkowski’s characters through the work of CD Projekt Red, a Polish software house that has been making “Witcher” games since 2007. The most recent instalment in the series, “Wild Hunt”, summoned up a sprawling virtual world in which the player, as Geralt, enjoyed astonishing freedom of movement, cantering on horseback from ruined fortresses to monsters’ dens and onwards to the bustling streets of medieval towns. Created without Sapkowski’s involvement and set during a time substantially after that of the books, the games nonetheless mirrored their universe of interspecies tension, political machination and grey moral choices.

In a medium not known for the quality of its dramatic writing, the “Witcher” games’ uncompromisingly adult writing, character work and world-building were lavished with near-unprecedented praise from critics and players. And not just them. The Polish prime minister Donald Tusk presented Barack Obama with a copy of the second game in the series, while his successor, Ewa Kopacz, dropped by the developers’ offices to congratulate them on their third instalment. Suddenly, “The Witcher” was big business worldwide. Earlier this year, CD Projekt Red was valued at $1.6bn.

All this must have been ever-so-slightly galling for Andrzej Sapkowski himself, a video-games sceptic who had signed away the rights to his books for a lump sum when the idea of making a game was first mooted. “I was stupid enough to sell them rights to the whole bunch,” he later said. “They offered me a percentage of the profits. I said, ‘No, there will be no profits at all – give me all my money right now!’”