Genshi Doumei
Also known as Alliance of Illusionary Eyes, a free downloadable short story by Nasu that takes place about two months after the events in Tsukihime. The game was originally released as part of PLUS-DISC in January 2001. According to Mirror Moon:
Prior the release of Tsuki-Bako, Kiyobei, the lead programmer at TYPE-MOON, wanted to shift from NScripter to a more versatile script engine (KiriKiri2), but was unsure as to how it would perform speed-wise, so Alliance of Illusionary Eyes was also released to the public as part of an evaluation test for the KiriKiri2 script engine in 2002.
Upon finishing the story, people were asked to complete a survey detailing their experiences in using the evaluation test, and from that point onwards, starting with Tsukihime Plus+Disc, TYPE-MOON started to use KiriKiri2 as their main script engine for future projects.