2007 - Sea of Darkness

The Toa Inika and the revived Piraka chase after the Mask of Life, which eventually plunges into the bay encompassing Voya Nui. Receiving a clue about a city beneath the ocean and conversing with the recently freed Toa Nuva, the Inika descend through the stone cord that keeps Voya Nui afloat in the hope of reclaiming the Ignika, while the Toa Nuva begin a set of tasks for Mata Nui's reawakening. Neither group has any idea what the Ignika has in store for them…

Whilst travelling down the cord, the mask transforms the heroes into the water-breathing Toa Mahri - complete with new masks and weapons - for their mission for retrieve the Ignika from the ocean within a time limit of 24 hours.

The Ignika's descent ends in the underwater city of Mahri Nui, where it becomes a beacon of hope for the Matoran villagers as well as the Barraki; six former warlords sent to the Pit - a prison  with a seemingly bottomless chasm that sits on the lip of Mahri Nui - after their crimes as part of the League of Six Kingdoms. But after the Pit was damaged in the Great Cataclysm, they escaped into the ocean and were permanently mutated by its waters. 

Believing the mask can change them back into their original forms so they can rule the Kingdoms once again, the Barraki and their aquatic armies battle against the recently arrived Toa Mahri and gamble for possession of the Ignika. 

Matoro is later imprisoned in the Pit by a robotic Maxilos guard that reveals itself as Makuta Teridax, who possessed the robot after his essence followed the Toa through the stone cord. He informs Matoro that Mata Nui only can be saved if the Toa shatter the cord. Not trustful in the villan's words, Matoro relays the information to his team-mates. They agree and begin evacuating Matoran from both locations before proceeding to destroy the cord that sends Voya Nui plummeting into the ocean towards its original position, demolishing Mahri Nui in the process. 

But by the time the Toa retrieve the Ignika, Mata Nui dies. Believing there's still hope, the Toa battle the Barraki's armies while Matoro races towards a hole in the seabed with the mask before Voya Nui descends to close it. Once in, Matoro falls through a waterfall. He dons the Mask of Life and uses its powers to teleport the Toa Mahri to Metru Nui and turn them back into air-breathers, before the mask turns his body into pure energy that saves the Great Spirit. Meanwhile, the Maxilos body that Teridax inhabited is found badly damaged after a ferocious battle with the Barraki, and is so presumed dead. 

Mata Nui has been saved, but remains asleep.