Back in 2015, CD Projekt released MODKit for The Witcher 3: Wild Hunt, a mod tool that "enables [the] Witcher community to modify several aspects of the game to their own liking." As we noted at the time, however, it's not the mod tool: That'd be REDkit, a far more powerful piece of software that lets users create entirely new things in the game, rather than simply changing up what's already there.
CD Projekt had previously released REDkit for The Witcher 2, but said it was not planning to release it for The Witcher 3. What a difference nine years makes: Today CD Projekt began taking signups for a Witcher 3 REDkit playtest on Steam ahead of a planned launch later this year.
