This Data Pack is required by Vampires Need Umbrellas for enabling the Umbrella Curios Slot.
Starting from 1.20, Curios changed the recommended way of registering and enabling new slots from code to Data Packs, so this was made to replace the old config file option.
Using this Data Pack together with Vampires Need Umbrellas will allow you to put your umbrella into a special slot instead of occuping your hand.
