The Complete Book of Villains (1994)

From the back of the book:

There is no little enemy. - French proverb

A paranoid warlord, a bitter drow priestess, a power-mad archmage, a sly dwarven assassin - not all the characters in the AD&D game went the way of the heroes. Villains - being dedicated to committing evil - are the most dangerous of all foes, and thus the most useful of all tools that a DUNGEON MASTER has to create a memorable campaign. This book presents the most complete guidelines ever offered on creating villains for role-playing games. Miss this advice at your own peril!