Arx Fatalis is actually really rather a cool game. Think Ultima Underworld, fully jazzed up and with a nifty story full of surprising twists. I might produce a review of it at some later stage.
Arx isn’t a new game by any stretch of the imagination, being more than eight years old, and it had persistent performance issues except in the lower resolutions and on some specific hardware. Nevertheless, a patch (1.21) suddenly appeared which fixes all of that – although it actually maybe feels a little fast now.
Apparently it is the final patch for Arx Fatalis, but it is accompanied by the source-code under the GPL 3 (with a couple minor exceptions).