The Toa/Dark Hunter War was one of the largest wars in the history of Metru Nui that took place roughly 2000 years prior to the events of the events in Bionicle Adventures. Although much of the detail has been lost over the years, we know a few facts. The War began when "The Shadowed One" "requested" that Turaga Dume allow him to build a Dark Hunters base of operations in Metru Nui. Dume refused, so the Shadowed One sent forces of Dark Hunters to attack the city and claim it as their own. The Toa Mangai who had come to Metru Nui to defeat the Kanohi Dragon fought valiantly during the War. Hundreds of Dark Hunters descended on the city and the Toa were driven back. All seemed lost until a Toa of Water, Naho, escaped the city and requested help from other lands. Nearly one hundred Toa arrived and the battle became a prolonged standstill. Much of the war was fought in an Urban-combat situation, with much sneaking around.
Another notable occurance during the war was the betrayal of the Toa by Nidhiki, who joined the Dark Hunters (after being shunned from Metru Nui by Lhikan). Nidhiki encountered Lariska and in the end the two made a deal, the Dark Hunters capture the city and Nidhiki gets to rule it. The next day, the two arranged for a confirmation of the deal and worked out plans, they planned to trap Lhikan and his teammates in a valley and kill them, thus gaining control over Metru Nui. Toa Lhikan, who had followed Nidhiki, overheard everything. Hakann, who was following alongside Lariska, but stood behind for the negotiations, spotted the Toa. Eventually, the Toa of Fire made a deal with Hakann. The deal was to allow the Dark Hunters leave safely in exchange for the Makoki Stone. Lhikan agreed, and later on, more than 300 Toa would surround the assembled Dark Hunters. Lhikan announced the defeat of the Dark Hunters, and they left, along with the now-banished Nidhiki.
The Toa's home base was the Coliseum in the heart of Metru Nui. It is known that there were as many as 400 Toa in Metru Nui during the war.