The Kanohi Ignika hails from the Matoran Universe which Mata Nui used to have control over. It was meant as a failsafe to both heal Mata Nui should something happen, and destroy the universe should evil be prevailing. It was used twice, by two Toa, to save Mata Nui. After Teridax took the body of Mata Nui, he forced Mata Nui's spirit into the Kanohi and launched it into space, where it landed on Bara Magna.
The Kanohi Ignika has powers over life. It can age both ways, rust, create decay, restore, use energy blasts of life, bring things to life, instantly kill/wipe things out of existence. It is a very deadly tool, but Mata Nui cannot fully control it. The Kanohi occasionally speaks to him and helps him with its own powers.