The Sixth
The Sixth was an odd fellow. He lived on a desert planet with a group called the "Brotherhood of the Sixth", who may or may not have worshipped him. He was also powerful, even more so than Ta'harok.
When Ta'harok was marooned on the Sixth's planet, the Sixth killed him, revived him immediately afterwards, and offered him help in his war against Blackout. Ta'harok gladly accepted, and the Sixth and his Brotherhood joined him in the Battle of Mata Nui II. Despite his considerable strength, he was severely injured by Blackout, leaving him out of action. He was taken out of the line of fire by one of Ta'harok's allies, which didn't do him any good. When Naju decided he didn't want to be on Ta'harok's side anymore and went on a rampage, the Sixth was one of his unfortunate victims.