Kraahkan

The Kanohi Kraahkan was the Great Mask of Shadows. It was made and kept in Artahka, until it was stolen by the Makuta.

When Makuta Teridax and Toa Takanuva merged to form Takutanuva, the Kraahkan and Avohkii also merged, forming a mask with the powers of both Light and Shadow, or unofficially known as the Mask of Light and Darkness. However, the two beings and their masks soon split apart again.

When the Piraka entered Mangaia, Reidak attempted to claim the Kraakhan from the shattered armor of Teridax, but it pummeled him with energy until he threw it into the Silver Sea.

Sometime after this, Makuta Icarax dived into the sea and emerged with the Kanohi Kraahkan. He then replaced his old mask with it.

Powers
It allowed the user to control the shadows and feed fear and/or anger into those affected by its power. It was made out of Protosteel, and could also give the user the ability to see the darkness in the souls of others. The Kraahkan also changed shape when its wearer shapeshifted - it was transformed to look like a Hau at least twice.

Trivia

 * In the Parallel Universe, the Kraahkan was displayed unguarded in the Metru Nui Archives as a trophy, and as an insult to Teridax.
 * The Kanohi, as seen in Makuta Teridax set and in Makuta Icarax set, could be turned upside down to make it look more fearsome.
 * A gold version of the mask was released in a playset.