2002 - The Bohrok Swarms

After their battle with the Makuta, the Toa emerge from his lair victorious, only to discover that the villain has launched a new threat to the island of Mata Nui: the Bohrok Swarms; six breeds of robotic bug-like drones designed to "cleanse" the island to its former barren state.

The Bohrok rampage across Mata Nui and begin tearing down the regions of each village, forcing the Turaga and Matoran to flee. And while many Matoran are driven under the influence of the Bohrok's brain-like Krana controllers, others find new ways to retaliate against the swarms. 

The Toa begin to collect all eight varieties of Krana from each breed while engaging in combat with the Bohrok and later reunite to travel down to the swarm's underground nests. There, they place the Krana in niches located in each breed's hive, but this alone fails stop the swarms. The Toa later discover and don powerful, sentient robotic battle armor called Exo-Toa which lead them to the chamber of the true swarm commanders; the Bahrag twins Cahdok and Gahdok. While engaging in battle against the nest Queens, the Toa learn that the Exo-Toa are hampering their elemental powers, only after taking them off were they able to re-access them and trap the Bahrag in a cage of solid protodermis (a material created by all six Toa's elemental powers). 

On the surface, the swarms stop in mid-action, and the Matoran begin the process of rebuilding their villages using the Bohrok drones now that their Krana are powerless. The Toa's victory is short-lived when the ground beneath them opens up and plunges each of them into tubes filled with energized protodermis, transforming the heroes into...something new. The protodermis mutates the six Toa into more powerful beings with new armor, masks and weapons and creating the end result -  the Toa Nuva! 

Emerging from the tubes and returning to the surface, the Toa Nuva test their new abilities by combating one another. Their actions lead to anger amongst them and they all think it best to separate and return to their own villages.