IOSYS is a group that does Touhou remixes, among other things. This song, Border of Death, is their remix of Necro-Fantasia. It's not part of their best Touhou CD, although it IS very good. (They also did the songs "Marisa Stole the Precious Thing", "Phantasmagoria Mystical Expectation", and "OVERDRIVE", each being notable in that they represent their CDs with flash movies, similar to this one.)
Marisa Stole the Precious Thing was my favorite IOSYS flash, but now that I've played the game this is based on (Perfect Cherry Blossom), this one has taken its place.