It's a simple path, you say. Nothing to worry about, and if you want to reach Fallarbor Town, you have to go through. The rumors about Slugma and the Poison-type Pokemon are fake; who would believe that? And besides, you have your trusty Marshtomp with you, and worse becomes worst, you can use your Shedinja's Dig to escape the place.
Entering the path, the first thing you notice the sweltering heat hit you in the face. You ignore it, as many trainers before have. Distracted as you attempt to sent out Marshtomp, you trip over a rock and the most catastrophic result possible happens.
You hit a Koffing.
"Oh sh-" is all you can say as you fall on it, causing it to release highly flammable poison gases. You vainly attempt to get the poison Pokemon to stop before you faint, poison circling through your veins. The mud fish Pokemon stands its ground, eating it's Pecha Berry. It uses Take Down, ramming into the Koffing and sending it flying.
Straight into a Slugma. Any living object would be unable to stand a few seconds in magma, and when said living object is filled with flammable gases prone to explode, the results are even worse.
You should thank God you're unconscious. The same can't be said for Marshtomp.
Koffing exploded, sending toxic gases everywhere, causing a blanket of flames and blowing the Fiery Path wide open. Said flames came in contact with the local Grimer population, setting them ablaze. Flaming toxic sludge was flying everywhere, and whenever a Koffing got hit, another explosion. Numel and Torkoal, due to the stress the were in, released powerful Eruption attacks, hitting more Koffing.
By the time all the Pokemon there had died, the equivalent of the world's biggest and most dangerous chemical reaction to have ever occured in the Hoenn region had just finished. Your charred body lay there as the fire began to spread, killing off the remaining Pokemon and reaching towns. There are no survivors, the explosions killing the Torkoal, Numel and Slugma, the flames killing Koffing and Grimer. A world ablaze, there is nothing you can do about it. It is your fault.
After the flames have been quenched, the result is shocking. Lava from the nearby volcano had started to flow out of the massive gash in the mountain, filling a pool of magma and sealing your fate.
[PokeDex R/B Koffing] Because it stores several kinds of toxic gases in its body, it is prone to exploding without warning.
[PokeDex Yellow Koffing] In hot places, its internal gases could expand and explode without any warning. Be very careful!