The party I am DM’ing is going for an adventure to the Elemental Plane of Fire. I want to design a magical weapon that can grant an advantage against fire-based creatures. I see that Fire Elementals have Water Susceptibility (Cold Damage) but I have already house-ruled that elemental planes can not include magical effects of elements from other elemental planes, but just the spells that have elements of the current plane can be cast (as the plane purely consists of element of itself). For example, in the Elemental Plane of Air, not only Earth-based spells but all the spells are impeded, except air-based ones.
After I realized that I can’t use a magical weapon that has cold damage due to my house-rule, I wanted to give the weapon a "heavy smoke" effect to choke the fire, as I thought that fire can not live without oxygen. But this option conflicts with my house-rule, also I saw that the fire-based creatures any other fire source in the Elemental Plane of Fire don’t need air to live.
Now I have no idea how can I design a magical weapon that gives an advantage against fire-based creatures without cold damage and choking the fire via leaving without oxygen is not an option. I can not cancel my house-ruling because of the sake of my setting. I am open to any house ruling ideas beside the official ruling.
TLDR: as in title, how can I design a magical weapon that grants an advantage against fire-base creatures without using water and choking effects?
If the question shows up to be opinion-based, please accept my apologies.