Given the variety of kings, factions and political intrigue throughout the world of The Witcher, it's only right that its universe be paired with a game that enables Machiavellian-style warfare.
It's pretty straightforward: it turns the Paradox grand strategy game into a battle of international politics, warfare and diplomacy with all the Witcher characters you know and love. The mod itself is set in 1274 and concentrates largely on the events in The Witcher 2, rather than Geralt's most recent adventure.
Images: The Witcher Kings (ModDB/Steam Workshop)
For added immersion, you can combine this mod with the Witcher 3 Music for CK2 mod, which is also on the Steam Workshop. But if you just want to start planning your wedding to Geralt and taking over Redania, head to the Witcher Kings listing here.