Mata Nui (Being)

Mata Nui, also known as the Great Spirit, was esentially the guardian of the Bionicle universe. He was looked to for inspiration by the Matoran, and he gave them the Three Virtues. He created many of the beings of the universe directly. He has an organization dedicated to doing his will, the Order of Mata Nui.

History
In the ancient past, the Great Spirit Mata Nui was sent by the even more powerful Great Beings to care for all life. He came to the Matoran people and gave them the Virtues of Unity, Duty, and Destiny. In this time, Teridax and his Brotherhood were some of Mata Nui's closest allies. Mata Nui also created several beings, such as the Barraki.

But Teridax became jealous of Mata Nui's status among the Matoran, and so hatched a dark plan. Invoking some unknown ceremony, Teridax placed Mata Nui into a deep slumber and tried to make the Matoran forget their Great Sprit and become their savior in his place, this event is known as the Great Cataclysm. As a result, Mata Nui could no longer maintain the universe as he once could, and the world began decaying immmediately afterwards. It was also discovered that Mata Nui was dying from the attack, and if he died, the universe would die with him in a matter of days.

But Mata Nui did see the attack coming, and manipulated events to make sure things could be made right. When new Toa Metru were about to be chosen by Toa Lhikan shortly before the Cataclysm, Teridax wished to influence the choices to his advantage. Knowing this, Mata Nui and his Order falsified prophesies regarding the identities of the new Toa. Teridax believed these prophesies, and influenced Lhikan to choose others to become the new Toa. But as these "others" were fake to begin with, the ones that Teridax influenced Lhikan to choose were in fact the true destined Toa Metru.

The Toa Nuva were sent to Voya Nui to find the Ignika, Mask of Life that could heal Mata Nui. Six Matoran followed them determined to help, and were transformed into the Toa Inika. The Toa Inika got the mask, after several struggles against rogue Skakdi Dark Hunters calling themselves Piraka, but it immediately flew away and plunged into the sea, heading for the undersea city of Mahri Nui that rested on top of the Pit, which it considered the most damaged place in the universe.

There, the mask was found by a Matoran named Kyrehx, who passed it on to Dekar. The Barraki went after it as soon as they learned of it, and the waters of the Pit were slowly destroying it. The Toa Inika pursued it down to Mahri Nui through the stone cord linking Mahri Nui and Voya Nui. They encountered a tribe of Zyglak, and a giant venom eel cracked the cord and attacked them.

However, they became Toa Mahri, transformed by the mask. They had from that point around 24 hours to heal Mata Nui before he died, and three days after that till the entire underground Matoran universe was destroyed. Unfortunately, they failed, and shortly after the Toa Mahri recovered the Ignika and cut the Stone Cord in preparation for Mata Nui's awakening, he died, causing the Mask of Life to go dark (it did not, however, lose its powers--the darkness was purely symbolic). The universe then slowly began to die; then Matoro put on the Ignika. Then, Matoro died, bringing Mata Nui back to life and the universe was saved.

Trivia

 * After Mata Nui's death, the universe would only have three days before ending. Islands on the surface would not be affected, but the entire underground universe would be flooded.
 * Mata Nui was actually not hard to find. The trick was knowing you had found him.
 * Prior to his death and revival, Mata Nui was, at least on some level, aware of what was going on in the universe.