My suggestions...
I would reccomend the following books.
Book of Vile Darkness: This book has a great deal of new necromantic type spells. It also has alot of material on Evil in gaming.
Ravenloft Campaign Setting: The 2nd edition had a system for "dark powers" checks, where every time someone did something particularly evil, they would risk gaining the attention of the Dark powers. I assume that the 3rd edition setting book from whitewolf has a similar system.
Call of Cthulhu D20: This book has plenty of rules that help you figure out what to do to someone when they interact with dark powers.
END COMMUNICATION