So I'm creating a silver Set that is supposed to deal more damage to undead. I want to do this using an entity Tag so that modded undead entities also work. So I created a json in data/forge/entities/undead.json:

{
  "replace": false,
  "values": [
    "minecraft:zombie",
    "minecraft:skeleton",
    "minecraft:wither_skeleton",
    "minecraft:husk",
    "minecraft:stray"
  ]
}

Here's the Code that should check if the Target is undead. (Don't mind that my Event's called differently, it does nearly the same as LivingHurtEvent)

@Mod.EventBusSubscriber
public class ItemTierHandlers {

    @SubscribeEvent
    public static void itemHitTierCheck(ItemDamageEvent event) {
        if (event.getItem().getItem() instanceof TieredItem) {
            IItemTier tier = ((TieredItem) event.getItem().getItem()).getTier();
            if (tier == ModItemTier.SILVER) {
                //Check for Tag and deal additional damage
            }
        }
    }
}

Now how do I use the Tag that I created above?

take a look at the vanilla EntityTypeTags (moj name)

EntityTypeTags uses bind() to create new tags. I don't find any way to get a tag out of my previously created json file. 

2 hours ago, FenrisFox86 said:

EntityTypeTags uses bind() to create new tags. I don't find any way to get a tag out of my previously created json file. 

the bind method needs an String which is the file name of the tag
minecraft then looks for EntityTags in tags/entity_types/string_in_bind.json in the data folder