Hmm...
To borrow a plot from a book I've read - would that be acceptable?
It seems like there's this major organization buried in the Russian government (rather like our NSA, or other, more secret, organizations like the National Defense Council) that came up with a master plan to kill all inferior races on the planet (yes, they were racist). They designed a genetically engineered piggy-backed virus that was no more harmful than the common cold. See, a piggy backed virus is actually two. One is attached to the other. In this case, the second, non-active, one buried inside the other was actually an incredibly deadly virus that would incubate for about 2-3 weeks, then cause death in less than 20 seconds. The first, active, one was a variant of the common cold, with it's mutative capability reduced - it would not mutate unless exposed to a very specific band of radiation. After exposing it at local airports, they estimated about 2 years for the virus to spread all around the world.
That specific band of radiation was caused by nuclear explosions in the air above the major continents. They really only needed a couple such for the virus to mutate and for the lethalality to activate. They buried themselves in a very deep bunker, isolated from the world.
3 Weeks later - the world was dead. The plot line diverges here from what you would want, but then, in the end, it comes back to the second half of what you might need...
Turns out they didn't think their plan was foolproof - it only had a 99% likelihood of exterminating all higher-order life forms (read anything that had a central nervous system of a sufficient complexity and some piece of the brain that only existed in primates - sorry monkeys!, but what an awesome cure for AIDS
Not! ). So they had a Plan B - A doomsday device in orbit. Nominally a space station, it was actually a titanic, heavily armored, planet busting nuclear bomb programmed to deploy if it did not continue to receive the deadmans signal from the deep bunker (deadman signal = signal only transmittable by live person). The opposition (from the part that doesn't apply) scrambled to try and stop this, or take shelter, which probably wouldn't survive.
Now the ending of the book doesn't match what you want (the opposition defuses the bomb during re-entry), however, with a slight 'what if' variation, assume the bomb hits, but for less than total destruction of the earth (the books 'best case' scenario for the 'what if the bomb hits' question)
500' rise in sea level + 1000' tsunami hitting every coastline in the atlantic and a 100-500' one in the pacific (bomb aimed to detonate at 50m above midatlantic ridge). Nuclear winter for 20-50 years, followed by a subsequent ice age (too much smuck in the atmosphere for sunlight to penetrate) that drops sea levels by over 1000 feet, and only neaves small, narrow bands of livable land near the equator. Ice age to last for 5 to 10 thousand years, during which levels of technology wouldn't support populations in excess of 100'000 for the first coupla hundred years, followed by an expansion of livable land as the ice began to retreat a little bit as the debris settled out of the atmosphere.
For your purposes, just play with the numbers a little bit. It's not quite a true nuclear holocaust, but close enough. Survivors would make it a legend, and point out the 'moving stars' in the sky, and the occaisional 'God Gift' of falling satellites to their children. I'd think it'd take more than 100 years, but it wouldn't take more than 2 generations before the 'old ones' drifted into complete myth.
You'd have some mutations from the virus (remember 99%?) and some from the nuclear winter. Let the elves come out, dwarves return, etc. as humans return to a more primitive lifestyle that doesn't necessarily exclude them. Let technology be incredibly anathemic to the humans, to the point of fanaticism.
Does that help?