I could see the Dragons dying out. It would be cool if one of the PC's were a half-dragon, intent on bringing them back. Also, I would see the Demons, Devils and celestials still around, but since worship has waned, so have they. They have no access to the Prime Material plane unless summoned ( in my games ). I could see the PC's unwittingly summoning a Balor, and he wanting to advise/ally with them to bring the living back....it would be boring to be a Ta'Narii and not being able to tempt with greed or power....I also see a few tribesmen doubting the abilities of the PC's, and plotting against them, because they think they are charlatans, or even moreso, are duping the tribes into a false hope, and will be working against the PC's. Maybe even 1 or 2 actually allying with Deaths agents to have the PC's fail, because they themselves have the same goals...not really Evil, but misguided...It would present a moral challenge for the Characters, what to do with them. You could have Deaths agents granting them power etc....I'd think roving bands of Giants, Ogres, etc....not mgical creatures, natural ones, could have left their Evil ways, and be struggling against the undead too....Maybe there is an exceptionally powerful Vampire Clan, which is feeding on the Frost Giants, in the mountains. Some of those obviously would have become undead also...Are you planning on using all forms of Undead, or mainly Skeletal ones....I mean Wraiths, Specters etc....I would say death raised up all forms from the army, not just animated ones....It would be cool to replay that as week 1, the denial of Havendash, battle of Ironwall, etc....then fast forward 1000 years....I think I'm going to go away from your idea ( which I like ) of the PC's knowing what has been going on this whole time. Perhaps they have been with their respective Gods since the battle, and the Gods have heard the call of the people, and sent the PC's back to aid them, since they were there during the battle. That fits more with my players, as it gives alot of roleplaying opportunities to learn about the last 1000 years...whew....I need to eat and go to work.....later...