The Witcher 3 is the latest and final installment in the Witcher series, which is a game translation of the fantasy book series by polish author Andrzej Sapkowski. The game series does not follow the exact story that the books do, but for the most part it has the same major events and characters. In fact, some of the characters are better/more often represented in the games than they are in the books. The games use a combination of a vivid world and engaging combat and npc interactions to bring the world of the Witcher to life. There are an endless amount of side quests not connected to the main story that help to bring out the world and characters, and all of them are unique. In addition, things the player character does while exploring the world actually have a visible permanent effect. For example, if the player clears out some monsters from a camp that used to be occupied by villagers, the villagers will then move back into that camp and start selling items and going about their daily lives again. This really helps to make the world seem more alive, not just a game.
The story is told from the perspective of Geralt of Rivia, who is the main character in both the book series and the games. Players can choose what Geralt says during most conversations, with different choices leading to different outcomes, some of which are relatively small, while others have an enormous impact on the story as a whole, even leading to different endings. Every player will have a unique story, even while experiencing the same events. Even while being marketed as a combat game, and having extremely good combat, the main purpose of the Witcher 3 is to tell a story. Specifically, it follows the main storyline of two of the books, and tells generally the same exact story, which it does exceedingly well. The differences mainly materialize in the very end of the game, where in order to have a successful game the developers had to change certain elements, but still manage to tell a compelling story. There are also some differences in the side quests, where instead of experiencing some events that happen in the books, the player sees and hears references to them.
The thing that really cements this as a great work of digital literature, however, is the fact that it managed to take a foreign piece of literature, and market it overseas. If it was just the book series, It's almost certain that it would never have gained this much popularity, but since the creators of the game, CD Projekt Red, did such a great job, the series is well known today. The most impressive thing that the authors did was the implementation of polish fairy tales as quests. The game changes them from stories told to children into serious and deep looks into polish culture, all while being displayed as a game storytelling device. Whether it was intentional or not, this provides an excellent look into polish culture.