create new damage types or properties?
Just wondering, is it possible to create new damage types and item properties? Let's say I make a monster that I want immune to most damage except a unique magic item, let's call it the Obsidian Hammer. When I create a monster, can I set vulnerabilities to a new damage type ' obsidian ' and set the immunities to' poison, psychic; bludgeoning, piercing, and slashing from nonmagical attacks that aren't obsidian ', then create the obsidian hammer and set one of it's properties to ' obsidian ' so that it will bypass the immunity and also take advantage of the vulnerability?
As I'm reading this, I think it may work as a 'property' but not as a damage type? I suspect I would need to set the vulnerability to the weapon damage type, in the case of a hammer Bludgeoning, but it would be immune to this bludgeoning damage unless the weapon also has the property ' obsidian '.
Is that right?