I am trying to make a new weapon deal damage to a custom mob, that can only be hurt with this weapon

Hello! I am trying to make a really big mod just as a passion project if anything. The mod idea is to add creatures that are unique, but can't be hurt through normal means. This is where I need help, I want to make a weapon, and enchantment, that are the only way to hurt these new creatures. Basically I am trying to make something like Smite, but in a sword by itself and only deals extra damage to these custom entities. Is it possible to make a custom damage modifier in this sort of way in MCreator? do I need to be using a specific plugin? 

I want to make a sword that is easy to craft, but doesn't last long, but can deal damage to these custom entities. but I also want to make an enchantment that works exactly the same, but on normal Minecraft swords. Also for the Entities, I want them to be immune to basically every melee attack unless it's the Custom sword, or a sword with my custom enchantment.

I hope this was enough info, and I really need help with this.