Man, I'm dying for that book!
As a DM, well, your NPCs can never be too powerful. I want god-like beings who can reduce cities to ash, raise entire nations from the dead to fight for them, change the landscape of the world with a wave of their hand.
Or, I should say, I've already got half-a-dozen of those in my campaign and have got no real way to justify what they're capable of rules-wise.
Which is no problem for me, really, but one of the ideas is that the characters eventually acquire this sort of power themselves. One of the central themes of the campaign is the "corruption" of power -- that those who possess extreme power will always be viewed as horrors by those who don't, regardless of what their actual intentions might be. And that in fact they WILL be horrors from some points of view.
My party is already proven to be capable of some pretty unpleasant things without thinking of the consequences. I don't even think they realise some of the things they've done in their quest to stay alive. Heh, heh, heh...