The Witcher (Polish: Wiedźmin, Polish pronunciation: [ˈvʲɛd͡ʑmʲin]), by Polish writer Andrzej Sapkowski, is a fantasy series of short stories and novels about the witcher Geralt of Rivia. In Sapkowski's books, "witchers" are monster hunters who (with training and body modification) develop supernatural abilities at a young age to battle deadly beasts. The books have been adapted into a film, a television series, video games, and a graphic novel series. The series of novels is known as the Witcher Saga. The short stories and novels have been translated into numerous languages, including English.
The books have been described as having a cult following in Poland, the Czech Republic, Hungary, Slovakia, Ukraine, Romania, Russia, Serbia, Bulgaria and other Central and Eastern European countries.
The video games have been very successful, and as of March 2018, they have sold over 33 million copies worldwide.
Source: Wikipedia, "The Witcher", available under the CC-BY-SA License.
The setting also has been adapted into tabletop roleplaying games.