The Darklands doesn't disappoint. The second entry in the D20 Lone Wolf RPG is a strong one. Beyond the information about The Darklands and Helgedad itself (the map is an nice inclusion), this book also includes a bestiary, AWESOME write-ups of Darklords (You'd better have a party of all 20th level before you run into one, essentially), and rules for corruption of characters who enter The Darklands, the book offers quite a bit more.
Evil character classes are also included in this volume. Now, it's one thing in Dungeons and Dragons to say, "My alignment is Chaotic Evil," but in LW evil is EVIL. The descriptions of the abilities, powers, and dark rites that evil characters go through will give you the shivers. Which is exactly what you're looking for in this book, of course.
Additionally, there's a "Tools of Evil" section that details various weapons, armor, and evil magical items one might encounter in the Darklands. All of it is, of course, deliciously evil.
The Darklands manages to equal its LWM brother in quality but it includes more great material.
1LR Review - 19 out of 20 - It's a Critical Hit!