First it is rare for a civilization to collapse from external forces. There has to be something happening that causes it to weaken. Civil war, natural disaster, factionalism, loss of resources, etc are all historical causes. So for a civilization the period proceeding the collapse is a bad time already.
Then during it's weakened state, external forces (monsters, barbarians, etc) can successfully attack the frontier and make their way into the heartland. Sometimes it even starts as being invited in either to help the civilization or to aid a faction in case of civil war. The tipping points comes when the aliens realize that the central authority have no power to stop them from doing what they want.
Now the result after various raids, looting, and assorted mayhem can go a variety of ways. New Nations could be formed, or the civilization experience a rebirth as result of two culture fusing together. The possibility of interest for a Points of Light setting is where the raids grow out of control to the point where it shuts down nearly all trade.
The invaders would be rich and powerful but only because they have been LOOTING everything not PRODUCING. In the situation we want for PoL they will turn on each other trying to grab the biggest slice of ever shrinking pie. This will continue until the survivors are forced to stop because of the lack of loot or strength and began herding, farming, again.
The end result is a land where civilization has been totally disrupted. Where there are a handful of settlement left from the old civilization. The surviving settlement can now survive because the remaining monster/barbarian forces can't muster enough to overcome even a simple motte and bailey. In theory they could if they united under strong leadership but in practice they hate each other as much as they do the civilized settlements. The landscape is littered with ruins of the civilizations largest towns. Some still exist but only as cleared spaces in the midst of the old ruins.
Another possible points of light setting is a land similar to Germany during the 30 years war. The surrounding kingdoms have fought so long and so hard over this one land that effectively becomes a wilderness where the monsters have returned. The surrounding kingdoms have lost too much blood for too little gain and have retreated behind their borders leaving the land to recover on it's own.
Yet another is a island or littoral based civilization and a huge volcano exploded or a massive earthquake occurred caused a huge tsunami to scour the islands and coasts inland for dozens of miles. Only the few villages on the ridges and foothill survive. The next several decades see a mad scramble for survival as refugees flood the few untouched regions. By the time some semblance of order is restored within the various villages the scoured lands have seen the return of monsters and unwholesome creatures from the deep. Thus setting the stage for our adventurers.
Enjoy
Rob Conley
