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The plan for my 4e campaign is to start as a normal medieval setting. The PCs are recruited to perform a mission of the knight of the manor which their village is a part of since the Knight and all the above-first-level PC classed NPCs in the village are presently engaging a band of Gnolls which have moved into the area of his lands.
(The mission is to find out why the creek flowing through the manor's land has suddenly all but dried up.)
During the trip up the creek bed each day they notice more and more meteors are appearing in the night sky. The numbers are increasing rapidly and by the third day (as they find the source of the creek's stoppage) the temperature has passed 100 degrees F and shows no sign of stopping.
The cause of the trouble with the creek is the fact that as the creek has come over a waterfall over the centuries, it has formed a pond (and caves beneath formed by the seepage through limestone, disolving it over time) which has collapsed into a sinkhole into the caves, diverting the water.
With the temperature reaching a dangerous level, the characters should be forced into the safety of the new caves and their cool air.
Since they do not know how long they may be trapped here, and with the constamt noise and spray from the waterfall, they should find it neccessary to go deeper into the caves to find a dry spot away from the noise of the waterfall to be able to rest and get dry and warm (with a fire of course, there should be enough wood deposited by the creek to meet their needs. I plan to let them find a magical satchel (wonderous item) which will instantly dry any non-living object put inside -- if they figure this out... thus giving them a way of drying wood for the fire as well as their possessions.)

I expect they should eventually explore deeper into the caves and eventually end up in a large cavern (an underground lake) lit from above by a chimney in the roof of the cavern where they will hear noises from above where they will be able to see crude bridges above the lake. (I am using the adventure from Dungeon Magazine # 84 page 66, heavily modified to use 4e Goblins and sans the earthquake.)

Once they deal with the goblins, they will be able to wait out the Meteor Storm and let the air cool (from its 250 degree F high) before exiting and either finishing their mission to restore the creek (they should be able to build a wooden "aquaduct" to direct the water over the sinkhole if they want to waste the time to do so) and/or treking back to the manor.

Only those who (or which) were able to get underground or somewhere which could stay cool for a week or so would be left alive, all else (including most Bacteria and Viruses which are killed at slightly below the 250 degree mark -- I had a good reason for choosing that temperature...) on the surface is dead.

Instant "Points of Light" setting where the PCs are likely those of the very few heroes left.

[The heating by meteor storm has basis in fact as it is currently thought the debris thrown into near-orbit by the "Dinosaur-Killer asteroid re-entering the atmosphere raised the teperature world-wide to something around 2000 degrees and is what reallty killed off the dinosaure and other life world-wide...]

I only have one problem now, how do I keep the underground races from totally wiping out the surface races now that I have killed off 95% of them?

Oh, I almost forgot, the meteor storm occurs every few thousand years, which explains (or helps anyway) why the races can't manage to develop past the medieval stage of technology. I also could use some help as to a way the fact of this cycle occurring can be discovered over the course of the campaign as the PCs try to rebuild society to a point they can be relatively safe from those races below.

Any suggestions/criticisms/discussion/(HELP) welcome.

Dan