Superitems

SuperItems lets you make custom items, armor, armor sets, mobs, bosses, loot tables, spawners, crafting recipes, NPC actions and regions without leaving the game, using either commands or a full GUI editor.

Items and mobs react to triggers like clicking, hitting, taking damage, equipping, shooting and dying, and each trigger runs commands with built-in logic (if/then/else, require, wait, cooldowns and variables) plus optional PlaceholderAPI support.

It also has a pixel texture editor, a built-in resource-pack host, undo history, and import/export, so you can build and share content entirely on the server.

Limitations: it's Paper-only, needs Java 25 and Paper 26.2+ (26.3 beta also works), and nearly every effect works by running commands, so it isn't a code-level API for other plugins.

Saving from in-game rewrites the YAML files and drops any comments you added by hand, mob trigger cooldowns reset when a chunk reloads. Note, not all functions are guaranteed to work 100% of the time. I would recomend checking my github wiki for more in depth information of how everything works. As of now, this version is version 3.0 of the plugin, wiki. FancyNpcs was used as the inspiration for the npc implementation, and Craftorithm was used for the crafting recipe system implementation. I highly recommend checking those out for standalone NPC/Recipe functionality.