[quote=@Temperance]
What kinds of enemies would our characters be up against? 
[/quote]

Synthesis has hit the nail on the head. Initially in the story, the 'enemy' so to speak wouldn't be victims of the Ashing as most have perished and those left are no threat to the immune characters. So this isn't going to be a zombie/cordyceps-esque apocalypse. 

The enemy early on will be more disasters, resulting from the collapse of society. The initial one I have in mind, is an industrial fire that threatens to burn down *insert town name* and if not stopped will burn through the entire valley and possibly beyond. 
I've also hinted at an incoming winter that will provide it's own set of challenges. 
Not to mention household pets gone feral and dangerous wildlife that is pushing themselves into urban areas.

However, I do fully intend to introduce NPCs and their respective groups that might not see eye-to-eye with our plucky group of players.