I kind of thought of the SW as a Ravenloft realm ( or very like). I like
the 'Legend of Sleepy Hollow" take on it. The adventure "Warlock of the
Stonecrowns" had a Shadow World crossover in it, that I rather liked.
IMC (KoGD), I have kind of dodged the "where do all of these undead come
from?" question, but I have given the PCs a taste when they found that the hag
covey had enslaved/eaten/animated an orog tribe. At another spot, they saw a
specter try to break out of its binding area, commanding ghouls.
I like the ideas suggested of undead as people slain in the SW, cursed to
keep coming back. Perhaps they come back as less (more?) powerful entities
each time they "die," but can gain power by forcing less powerful beings to
submit, or by entrapping/slaying live persons in the SW.
Hmmm. I guess I'll have to prep an adventure in which the party gets
caught on the other side, now! Gotta go!