Alkali Fever (5e Disease)
From D&D Wiki
Alkali Fever is a collective term in D&D Homebrew for the symptoms related to smelling molten material that a normal creature can't withstand. This is common practice in the existing examples of D&D diseases, such as Sewer Rot, and can therefore apply across multiple causes.
When a creature starts its turn for the first time in an area where the air has toxic metals and other elements in it, and every hour they remain in such a location, they must roll a DC 15 Constitution saving throw and are infected with Alkali Fever on a fail. When first infected, the creature gains 2 levels of Exhaustion and gains another one every hour until the disease is cured. They are poisoned until the disease is cured. Other symptoms exist as well, such as nausea, vomiting, excessive thirst, abdominal pains, etc. These symptoms manifest by forcing any attack roll, ability check, or saving throw to act as a critical fail if the creature rolls a 5 or lower (6 or lower if they normally critical fail on a natural 2 for any reason). When this occurs, the symptoms mentioned above overcome the creature, preventing it from moving or taking any actions for a round and dealing 1d12 Necrotic damage to it.
The easiest cure for the aliment is to escape the area, breathing fresh air. While not spending time in an area that causes the disease, they lose one Exhaustion point each hour (But only those gained by the disease's effect). When they no longer have ANY Exhaustion points, at the end of an infected creature's turn they can attempt a DC 15 Constitution saving throw to end the disease. Spells that end disease do not cure the Exhaustion points unless the spell explicitly states that it ALSO cures Exhaustion points. If cured by a spell that doesn't remove the Exhaustion points, the creature loses one Exhaustion point each hour until they are all gone.
DM Note: Nasty little piece of work, this one was. Can't really imagine it'll be all that fun for normal use, but it's realistic, with symptoms based off of Phosphorous gas exposure (Yes I know that's not Alkali, I just used that name because it's cool and generalized). If playing a "hardcore" survival campaign, this is for you. Also, note that while it's based off of a specific type of chemical, it can be repurposed for any type of molten or chemically infected air. The symptoms like nausea and vomiting are flavor text intentionally, and the infected can be just as overcome with other types of symptoms you use in their place without changing how the disease fundamentally works. Depending on the chemical reagent, DMs may also add new terrors to the mix, such as Paralysis that occurs within a round of being infected.
Wiki Note: Re-edited to remove unnecessary text and reorganize the page.