Karzahni was the legendary ruler of a domain much feared by Matoran in the dim past. According to ancient tales, in the time before the Great Spirit, Matoran labored in darkness. Those Matoran who excelled at their work were sent to Artakha and allowed to work in the light. Those who worked poorly were sent to another place, one ruled by Karzahni, from which they never returned. The name "Karzahni" eventually became a term Matoran used to scare each other, with each new tale about him becoming more and more exaggerated and frightening.
Karzahni was the twisted ruler of the realm also named Karzahni. He was powerful, almost regal, and quite insane. He was unkind and unmerciful. He ruled the realm of Karzahni for more than 100,000 years. He was a huge figure, with battered and twisted armor. His mask looked like four different masks welded together. His armor was black and gold and studded with sharp blades. He had gauntlets which cracked with energy. His eyes were deep and black and gleamed with madness. In one hand he carried a length of burning chain, his preferred weapon. Although he was very strong, he almost never needed to strike an opponent thanks to his power to make others see nightmarish visions of the future. These hypothetical scenarios usually focused on the horrible consequences of a failed attempt to escape, or some other event, in order to crush his victim's hope. Karzahni used this power to sow doubt and hesitation in those who opposed him.
Because of his isolation from the outside world for almost the entire age of the Matoran universe, Karzahni knew very little about anything outside of his realm. He had no knowledge of the existence of Mata Nui, the Brotherhood of Makuta, or Toa until he encountered Jaller and his companions.
Karzahni carried a flaming chain and wore a mask that looked like four masks melded together. He had the power to make an enemy see a vision of an alternate future, normally a nightmarish one.
Karzahni could cause his chains to burst into flame at will, even underwater. He could hurl these chains to ensnare foes and yank them off their feet. He claimed that not even the legendary Manas were able to break his chains. He was emerald-hued. He had keen hearing. He hated Artakha and anything associated with him.
Karzahni did actually intend to repair the Matoran sent to him, but he was poor at doing it. Like many frustrated artists, after a while he started blaming the medium instead of realizing the problem lay within.