CD Projekt RED has always been supportive of player mods. Even in the original Witcher they held a mod contest and then released said mods with The Witcher's enhanced version. Now they're releasing a modding toolset called REDkit in early 2013 to let players to both mod and create new content for The Witcher 2: Assassins of Kings.
The tool is reportedly in a closed beta at the moment, but you can rest assured that over a hundred modding terms are rigorously testing it for release. CD Projekt RED released the video below today that shows how a mission is created. The video is timelapsed, but it only took 23 minutes to build that specific mission. It's very cool to see things go from a flat grey plane to full environments with weather, vegetation, buildings, and monsters.
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I'm personally a fan of modding and very excited to see what talented people come up with for the game. What about you?