Kara no Kyoukai is a novel written by Kinoko Nasu (the man who wrote the script for Tsukihime and Fate/Stay Night and generally had the idea which started them) before he was known for creating Tsukihime. It's set in the same world as Tsukihime, though as usual there isn't much overlapping story content, which is somewhat of a shame in my opinion, since I wouldn't mind a few character cameos once in a while. Mirror Moon has graciously translated a few of the chapters of Kara no Kyoukai on their homepage if you want to read it in English, actually.
"They marched onwards, fire against earth, bodies piling ever higher for the whims of rival demons, content in their game. The masses never questioned their intent nor knew their masters, serving Hell more than Heaven in their madness. For madness went unchallenged here, and the great machine of many faces, skins, and thoughts continued to turn in silence."