Although it's based on the d20 system (shudder), it's got a good bit of information for inserting an underdark system into any game world (or just using the underdark AS the game world). I'm currently starting out a few PCs 'below,' and this has given me a great deal of info and inspiration to start fleshing out the world, for more fun and excitement, once they leave the city they're currently adventuring in (I planned it that way, as I needed to create more of the world before I could get them out and about).All in all, it's a good book, although a little more system-independence would've been nice.
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