If you like the grand strategy of Mount & Blade 2: Bannerlord, and lets you fight the battle there. Then it sends the results back and you pick up your CK3 campaign right where you left off.
It's a wonderful idea, and looking at the videos, seems to work surprisingly well. The massive clashes possible in CK3 have to be represented proportionally by the smaller forces of M&B2, but then, one archer in a Total War battle clearly doesn't represent one human being either.
For the switch from one to the other to work properly you'll need to select a maximum battle size, set Mount & Blade 2 to run in windowed or borderless fullscreen mode, and make sure the mod knows exactly where both game's executables are. You can Crusader Blade from itch.io, and its creator has a Patreon. Future updates will apparently ensure it's compatible with DLC.
Thanks, Kotaku.
