The Fall in DUST occurs like this, pretty much: The Courier wins the battle of Hoover dam, (Independent/House ending) but the NCR apparently oust him or something and take over Vegas. They evict the people of Vault 21 and they do some deep experiments there. A worker sabotages the place, and the experiments escape. But the NCR dump radioactive waste to prevent them from getting out/people knowing what happened. After that, they occupy the Lucky 38, conduct more experiments. While that's happening, the Freeside Kings decide to rebel against the NCR, this leads to violent uprisings which ends with NCR victory. The Kings and the other rebels hide in the sewers though. Later, they escape to Boulder City, and the sewers are occupied by cannibals which lure survivors in by saying they got food, etc and eat them. The King gets shot and dies in Boulder City. Pacer and the rest apparently get mutated to Ghost People after they wear the suits in Boulder City.
Meanwhile, they (the NCR) discover the Sierra Madre, but Elijah is hiding there. They use tribals to scavenge tech. Elijah manipulates the tribals and gives the NCR only what he wants them to have. After a while, he decides to give them the cloud.
Shortly after that, a sandstorm hits the Mojave. A really bad one that also brings plagues. (one of those plagues wiped out Goodsprings entirely) A while later, the Cloud explodes in the Lucky 38 and the entire Strip area becomes Sierra Madre V 2.0, but with Marked Men, Ghosts and Feral Ghouls. Also Centuars, but those were made by the NCR. (one Centuar has a note which says he was a NCR soldier)
The BoS are killed when the NCR drops off radioactive barrels, killing them all Camp Searchlight style. Some people who were out (led by Paladin Ramos) took a new base in Camp Searchlight, and they find out that Elijah was the main cause of everything.
In Old World Blues, everything is dead, but apparently the Courier activated all the Sink AI stuff. The Toaster has a blood shrine and the Sink declared war on filth -- which means that it literally flooded the entire crater. (not kidding here.. the only areas you can walk are the high/elevated areas) There are 2 factions. A BoS detachment and tech scavengers. both are hostile of course. OWB is accessible through the Nipton tunnel, but you need C4.
In Honest Hearts, Joshua Graham gets infected with a mutated Vault 22 Spore thingy (Brought to you by the NCR accidentally). He dies, and is buried. Shortly after that, the spores take over the entire canyon. There's a thick green fog. Later, a new mutant appears. People call it Wendigo, it's unkillable and motherfucking annoying. It's a huge Spore carrier. HUGE. It walks, it stretches itself in weird poses and is the worst enemy in the entire mod. Daniel who is alive believes that that thing is Joshua Graham, he sets out to kill it and goes missing. There are some NCR troops who went insane due to the fog, the spore carriers, the spore plants and of course Wendigo. HH is accessible through the cavern. Wendigo is afraid of fire/light btw. I strongly advise you not to enter this area unless you have a Flare gun or a flamethrower or something like that. You will never be able to kill Wendigo, but you can at least keep it away a bit with those.
In Dead Money, there are still ghosts and stuff, but you can find the skeletons of Elijah and the tribals. You can go to NCR territory through here. DM is accessible when you take the Vertibird from (former) Camp McCarren. The Vertibird crashes in the Sierra Madre.
Lonesome Road is fairly empty. Tunnelers mass migrated to the Mojave and the Marked Men are scarce. There are some neutral people here, too. The White Legs escaped to here after being chased by the NCR and the cloud. They found Vault 74 and wiped out everyone in it. When they got out, the Marked Men had made an alliance and killed every single White Leg.
That's it pretty much. It's literally Fallout: Everything-Goes-To-Shit. It has well done lore though. I am interested in FROST, which is FO4's DUST. It apparently takes place shortly after the war. Waaaay before FO4 itself.