I've decided on having the Kingdom of Zagadur collapse 20 or so years before game-start. This will provide me with a relatively fresh 'layer' of history, complete with its heroes, events (some of the older NPCs will have them in their backgrounds) and adventure locations.
The question is, however, what has brought the end of the Kingdom. I have several possibilities in mind:
1) Infighting between the various Counts and/or Barons leading to a fully-blown civil war, possible after an unclear case of royal succession; essentially everyone tries for the throne and end up destroying each other in a massive bloodbath. This is the most obvious idea, but not necessarily the most interesting.
2) A prolonged war with an outside force (the queendom of Korovod? Goblinoids? Snow Orcs?) eventually destroying the kingdom.
3) As #2, but with an outside force (Goblinoids? Snow Orcs? Mammoth-riding Barbarians?) actively sacking the kingdom and then disintegrating, politically speaking (I want a fractured sandbox, not an occupation situation in most locations).
4) Something more ominous (a high-level spell gone wrong? a plague of undead? a normal plague?)?
Ideas will be welcome
EDIT: I'm leaning towards a coalition of monsters (including goblinoids and orcs) attacking the Kingdom and eventually defeating it, possibly led by a villain (a particularly smart goblin or kobold? a mind-flayer? a mage or necromancer of some kind?) and the falling apart soon after due to squabbles over the loot. The larger cities will be sacked; some smaller towns and villages will remain free.
EDIT #2: It has occurred to me that I can probably combine options #1 #2 and #3 (and possibly even #4) into one story, with the monster alliance being just one of the factions in the Zagaduran civil war, possibly arising to exploit the fact that the Kingdom of Zagadur is weakened by infighting...