Atza

The Kanohi Azta is a mask that used to belong to Kulnak, but was destroyed along with his original body by TMV.

Its power is unknown, but considering Kulnak's obsession with poison, it can be assumed that it is poison-related. This would also make it an immoral mask unlikely to be used by Toa, which would also explain why only a Makuta has been seen wearing one thus far.