Part of Devil's Workshop's line of single prestige classes, the Obsidian Warlock is a great foe just waiting to challenge your PCs in the dark, when they least expect it. And if it's not dark enough, he'll make it so. Players step aside; GMs, this is a PrC just for you.
From the PDF: "For the Obsidian Warlock, the night is a thing of beauty that holds mysteries other mortals fail to fathom. It is through a clearer understanding of the shadow essence that the Obsidian Warlock acquires what he truly desires: Power."
The mechanics of this class are great and really tight, supporting the concept from start to finish. Entry requirements are well chosen (though I'm not sure why there's a BAB requirement on a magic-based class), setting the mood for the class from the start, and allowing for some minor customization.
The class abilities are where this bad boy stands out. In order for the Obsidian Warlock to really come into his own he needs darkness, and if it's not natural, he'll make it. Once he has that advantage, he's good to go with excellent shadow-based abilities such as Shadow Casting and Shadow Manipulation. He also gets some emotion-based powers, drawing on the primordial fear of the dark, that complement the shadow-based powers quite nicely. The only strange thing I saw is that some abilities last a number of rounds based on the Obsidian Warlock's Will save. As a GM you should figure out if this will become too unbalancing, and perhaps adjust it to prestige class level + Int/Wis/Cha modifier (depending on the Obsidian Warlock's main spellcasting stat).
Though I could see a PC taking this class, I think it is better suited for NPCs (either friend or foe, especially foe) adding a nice touch of mystery to the campaign. For .99¢ you pretty much can't go wrong, adding a very interesting tool to a GM's bag of tricks.
Now go make a Obsidian Warlock/Shadowdancer to torment your players with.
