Custom enemies are a major addition to Mari0: SE. Like the name suggests, it allows users to add new enemies to the game. Instead of coding enemies directly, custom enemies are created by inserting and setting several properties using JSON.
Each custom enemy is created with one JSON file, which is the same as a text file, and one PNG image (although if the enemy has a base then the PNG is optional).
All vanilla enemies were converted to the custom enemy system except for Bowser. However, there are some issues with the conversion.
Pages to help authors create their own Custom Enemies can be found in the Custom Enemy Documentation.