The Divine Relic charm makes it so your weapon is randomly changed every time you fire, but it normally can only pick from the 9 default weapons. This mod makes it so the relic also includes any custom weapons you have installed using the Blender API.
NOTE: You'll need to have another custom weapon mod installed for this to change anything. Installing this mod by itself will not change how the Divine Relic works.
Extract the zip in C:\Program Files (x86)\Steam\steamapps\common\Cuphead (or wherever you have Cuphead installed)
Equip the Divine Relic (or Cursed Relic) and it will randomly select any custom weapons you have installed in addition to the base game weapons.