In version 1.7.2 ( Royal Court DLC era), you can create artifacts to display in your court or wear as equipment.
If you want to create a new "piece" of the game itself (like a mod), use the CK3 Launcher : Open the . Select All installed mods on the left. Click Mod Tools at the top right, then Create a Mod . Fill in the Name, Version (e.g., 1.0), and Folder name. Crusader Kings III [v1.7.2]
: Right-click the game → Properties → General → Launch Options → Type -debug_mode . In version 1