kagamination ZERO is the prequel of the compilation album Kagamination first distributed on May 8 (in the event Kagamine Now / 鏡音なう！). It was intended to be a "large" compilation with over 40 producers, but due to the earthquake incident on March, composers and illustrators couldn't meet the deadline.
Later, kagamination ZERO was released with the tracks "as is" with 6 new tracks and a 4 pages booklet. On Kagamination, the full tracks and booklets (with over 30 songs) was released in June.
It's still available for purchase in Toranoana.