[Region 1][Map 04] Cazgothe

[waldronate]

How about a peculiar racial reaction to the plague causing rapid spread with universal infection combined with very high mortality (think on that spreads like the common cold with an AIDS-like disease progression but faster decline after the symptom-free state). Death happened too quickly and/or in a manner that many of the spirits become sundered and stuck in the world?

All it would take is a 6-month incubation/infectious period followed by rapid symptoms and death. By the time anyone noticed that they were sick they'd be dead and the whole population would likely by infected by that time. All the people gone with just spirits left to terminate any newcomers. And best of all, no real chance of new plague outbreaks with adventurers because everyone who was symptomatic is dead now.