The Dark Time
The Dark Time is the name given to the thousand-year period following the Great Cataclysm, during which the Great Spirit was asleep, and Visorak hordes led by the Brotherhood of Makuta
In this time, the populations of Metru Nui and the region surrounding Mount Valmai were displaced to the surface of Aqua Magna, inhabiting the islands of Mata Nui (Location), Voya Nui, and Mahri Nui. The populations of all three islands, cut off from the rest of the universe, were in desperate need of Toa, and their calls were eventually answered by the Toa Nuva and Toa Mahri.
The era came to an end when the Toa Nuva carried out the instructions laid out in the Scroll of Preparations, traveled to Karda Nui, and resuscitated the Great Spirit. However, they were deceived, and when they reactivated the Great Spirit Robot, it was Makuta Teridax's mind in control of the universe. This began the brief period known as the Reign of Shadows.
Background
Makuta Teridax, as part of his plan to become the new Great Spirit, uploaded a virus into Mata Nui's processors, which deactivated his functions and sent him into a deep sleep. Unable to perceive his surroundings, Mata Nui's body, the Great Spirit Robot, crash-landed onto the planet of Aqua Magna, where the resulting collision caused death and destruction on a massive scale for the beings inside his body.