If you like fooling around with some of the, let's say, less-historically-accurate elements of medieval dynasty management sim, Crusader Kings II, then a new mod that brings Geralt of Rivea to the scene will be right up your alley. More than that though, it converts the game completely into one built around The Witcher universe, adding all sorts of elements to the game and overhauling the gameplay in more ways than one.
The Witcher of Kings, is a total conversion mod for that game that brings characters from the games and books into the game world and turns that world into one dominated by the politics and intrigue of the Witcher universe.
Overhauling the game further are new additions like more detailed character focus options, plenty of new events with the supernatural elements that so define The Witcher. Some of those elements to build upon feature that are only available to DLC owners, though they aren't required for the game.
If you want to ahve the full mod experience, the developers recommend that you also own the following DLC:
- The Old Gods
- Sons of Abraham
- Conclave
- Horse Lords
- Monks and Mystics
However, the developers do caution about not downloading (or not installing at least) the Jade Dragon DLC, as it has poor integration with the mod and could break it.
If you want to give The Witcher Kings a try, you can download it from ModDB or via the Steam Workshop.