As the Matoran prepared to journey to Metru Nui, the Turaga (village elders) revealed that they had once been Toa, fighting in Metru Nui against threats such as the Morbuzakh plant creature and the Dume-controlled Vahki enforcement squads. They, as Matoran, were transformed into the Toa Metru by Toa stones containing power sacrificed by Toa Lhikan. Their mission was to find the six Great Disks and defeat the Morbuzakh, a plant menace that threatened to destroy the city. When they did, however, they were betrayed by Dume, the Turaga of the city. Later, they discovered that Makuta Teridax had taken the disguise of Dume and taken control of the Vahki (law enforcement robots). His plan was to put the Matoran to sleep, wipe out their memories, and reawaken them "as their conqueror." The Toa fled, but were attacked by Teridax. Combining their elemental powers, they were able to seal Teridax in a prison of crystallized protodermis. The Toa traveled to a new island, which would later be known as Mata Nui, vowing to return to save the Matoran.