Alright, I'm just gonna write what my au will be like, also I'm not going fully logical on this, because Im doing it for fun ^3^
SO! Why I named my au Grintale?
Because everything is falling apart, including the underground, minds and moral system in there.
Actually it's because the effects of CO poison that cause hallucinations and confusion making monsters to be crazy and sans is grinning so why not?
Why it's happening and what's going on there?
I'm gonna be honest, I have no idea why the kids went to the mountain, low-key they accidentally fell.... Ig they are all clumsy O_o oh and 'm not a fan of chemistry, so I would like to apologize to any chemistry people for my mistakes.
At the surface, there's a high Carbon Monoxide surrounding the forest of mount Ebott, caused by the humans as they burn off fuels like gas, wood, oil in their daily life. The burned that it reproduced, went to the forest as it thickened the CO, making a thick heavy fog. As CO were reproduced more and more, it crawled to Mount Ebott hole and the fog went down to underground.
Humans avoid the mountain now out of fear, or maybe they're unaware of the danger which makes Frisk's fall even more intense... Cus the forest felt lifeless yuh!!
The effects of Carbon monoxide starting from the Ruins until New Home↓
Since it’s the first underground area, it would get the earliest low-level exposure.
Toriel might notice her fire magic doesn’t work as well or burns with a strange color. Monsters with low HP or breath-based attacks (like Froggit or Whimsun) feel weak or act strange. The atmosphere is heavier, colder, and monsters get tired quickly.
Sans suspects something was wrong when he visited 'the old lady' behind the ruins, As toriel told something strange happened in the ruins, some monsters fainted for unknown reasons but some of them never woke up after fainting for days.
The cold could slow down the CO buildup a little, but it would still sink in over time ig? CO is heavier than air, so it settles low maybe in the valleys and snowy dips of the forest...
Sans, who always notices everything, could hint something is “off” about the air. He’s breathing, but it doesn’t feel like breathing. Funny how a skeleton doesn't have a nose to breathe..
Papyrus could be more hyper than usual ig? maybe from lack of oxygen, ironically.
Being wet and cooler, it might resist CO for a while.. But!! Caves and lower levels could have dryness, and air where the gas just sits.
Imagine a monster in a deep part of Waterfall who’s gone strange or ghostly from long-term exposure to the gas or maybe a corrupted monster who attacked their own kind cause by hallucinations and dizziness by CO poisoning.
This is where it gets really confusing for ME.
The lava and high heat would make CO combust if there were oxygen small fires or spontaneous bursts that appear. Alphys’s lab might be full of warning signs and maybe she's running experiments on the gas and trying to contain or filter it.
Oh and Mettaton could be powered by something that reacts with CO, adding more drama as if the gas is making him unstable or too powerful.
The CO gets there last or then, it becomes something more than just gas but a symbol of corruption, memory loss, or decay.
Asgore can feels the weight of the fog without understanding it. His fire magic is flickering and unstable.
Monsters could begin to vanish or act strangely, like their very code is degrading. (Could be checked in the multiverse)
Monster Reactions to CO Exposure
Weaker monsters faint or fall asleep and never wake up. For more sensitive monsters might hallucinate, lose memory, or go berserk, especially those tied to emotions or magic. Ghost monsters might actually 'feel' the gas differently, like it’s tugging at their soul-code or making them hear things.
Ok woah.. let's take a break AYO? (ʘᗩʘ’) Anyway idk if it's logical enough but it's fine, I don't need to bee specific about my au (@_@)