Reply to post #16
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Death explains this towards the end of "Hogfather" -- if anywhere else, too, I wouldn't know.
I think the book can be read like a supernatural mystery as far as they are concerned, though ... trusting that who they are and what is their role in the grand scheme of things will be explained when necessary; if nowhere else, in the final wrapup. They worked rather well for me as the mysterious shadowy bad guys in the background when I read "Hogfather" for the first time, not knowing anything about them.