Is there anything special I need to do to spawn my custom entity in the end?

I can let it spawn in all overworld biomes without a problem.

 

BiomeLoadingEvent with mesa biomes and end biomes as example, the entity will then spawn only in mesa biomes, but not in the end

@SubscribeEvent(priority = EventPriority.HIGH)
public static void onBiomeLoading(BiomeLoadingEvent event) {
	if (event.getCategory() == BiomeCategory.MESA || event.getCategory() == BiomeCategory.THEEND) {
		event.getSpawns().addSpawn(MobCategory.CREATURE, new SpawnerData(ModEntityTypes.BABY_ENDER_DRAGON.get(), 100, 2, 5));
	}
}

 for testing i registered a custom SpawnPredicate that returns always true

does you code work?

except that the mob is not spawning in the end, everything works the way I want it to

does it spawn somewhere or does it not spawn at all

in end biomes it does not spawn at all, checked with /kill by entity type

is the EventHandler called?

try to with different values

the eventhandler is 100% called, because the entity will spawn in overworld biomes when I change the category check

all overworld biomes seem to be working fine