A Shadow Matoran is a Matoran who has had all of his/her Light drained out, making them corrupt and evil. Currently, only the Shadow Leeches are known to be capable of doing this.
Any type of Matoran can be mutated into a Shadow Matoran, and once it happens, the Matoran loses its natural Element-related ability. The Matoran would also lose its natural colors. If a Shadow Matoran transformed into a Toa, it would still have its shadow element.