I believe the first game is Overture, well its the first one I played anyway. Its a mix of wonderful horror, mystery, physics even better than Half Life 2, and Resident Evil. I love the interaction that goes into the games. You don't just click on things to open, bash them open etc. You actually use the mouse in a Wii like way, but it ends up feeling more immersive as the sensitivity to motion is much greater with the mouse, thus you feel more connected. At least in my opinion.
Black Plague takes a new approach and gives you no combat options. In Overture, it was a bad idea but you could do it if you wanted to. Black Plague doesn't even give you the option, you go out of the shadows and you decide not to be careful, you die.
The games can be had for very little as far as I've seen. It runs and looks great on even the lowest spec machines. (I have a Apple PowerBook G4 12" which meats the minimum requirements by an inch and it still runs wonderful. Just turned off motion blur and was treated to wonderful graphics and frame rates). Overture is available for purchase new from Mac Game Store for about 19.99 and the PC version goes for around that as well. Its an awesome game and you should all treat yourselves to it.
The story is absolutely amazing. It was even awarded by some British Literature club as best horror story or something along those lines.
Again its not so much the graphics themselves that are great, its the atmosphere they create. It's just absolutely wonderful.
Be warned, the games are in fact frightening. Best horror game out there as well as adventure.
EDIT: I forgot to mention that after Overture and Black Plague there was Requiem that came out around August 2008 but I've never played or even seen it so I can't comment.
Now for some screenshots:



