What this does is take some parameters from Crusader Kings III ahead of a battle and export them to Mount & Blade II: Bannerlord so that you may actually fight that battle yourself with the action combat system featured in that game. The results are subsequently fed back to Crusader Kings III, allowing you to resume the campaign there.
Here's a more detailed explanation by the mod's creator, George.
I am a long-time fan of these two franchises and I really like the deep state management in Crusader Kings and the Mount & Blade combat system, and I thought why not combine them, taking all the best of the two games.
The basis of the modification is Crusader Kings III, the gameplay remains the same, except for one detail, now you can join the battle at any time. After you join, Crusader Kings sends parameters such as the number of troops, the militancy of the army commander (game character) and other warriors, the types of troops, the type of terrain, and others to Mount & Blade through a special gateway. Based on this data, a map is created and troops are placed in Mount & Blade. After the battle, Crusader Kings gets through the gateway the number of killed in the player's squad and his enemy.
At the first stage, the types of troops from Crusader Kings will be selected similar to Mount & Blade, at further stages of work on the modification, it is planned to create troops equivalent to those from Crusader Kings in Mount & Blade. The same applies to the cultures of the factions and the matching of their troop types. It is also planned to introduce the functionality of sieges and participation in tournaments and duels in the future.
You can take a look at Crusader Blade via the launch trailer below. There's also a Patreon that you can donate to if you wish to speed up work on the mod.