Someone wanted the code I used, so I uploaded it here in case anyone else would need something like this.

This is a simple utility data pack you can use to give your custom bosses (or any entity you wish) a custom boss bar (displayed on your actionbar title) which will dynamically react to damage inflicted to the boss, reducing it in a gradual, animated manner, akin to the behavior of a boss health bar from Dark Souls or Elden Ring.


Use this command to summon a test entity for you to try right away, after installing the data pack:

/summon skeleton ~ ~ ~ {Health:100.0f,Attributes:[{Name:"minecraft:generic.max_health",Base:100.0f}],Tags:["designated_boss"]}


Further guidance & comments present within the main function of the data pack.