Karzahni is one of the villains in BIONICLE. Although he serves as one of the villains,. He was the insane brother of Artakha and the ruler of the hellish land that shared it's lord's name.
Whereas Teridax was a being who was pure evil, Karzahni himself was as amoral, apathetic and vindicative as him.
Around 100,000 years ago, Karzahni was created alongside his brother, Artakha by the Great Beings. Soon after their creation, the two beings fought for the Mask of Creation. Karzahni was lost when he was defeated by his brother who claimed the Mask of Creation and he was banished into the island, which he created and named it after his name.
While Artakha chose to make a heavenly island for good Matoran workers, Karzahni chose to take in the bad ones, intending to repair their faults and return them to their homelands. However, his ability to fix the Matoran was not up to par, and he completely rebuilt the Matoran, but into weaker and hideous forms. To compensate for his inability to properly repair the Matoran, he gave each of them weapons. He then sent them to the Southern Continent to make sure his failures would not be discovered.
The Turaga eventually noticed that Matoran sent to Karzahni never returned from their exile, and stopped sending their poor workers. Angered by the lack of Matoran coming to him, Karzahni went into denial, blaming the Matoran for his failures. He refused to send any more Matoran away and trapped them within his realm, replacing their masks with his own twisted designs, stripping them of their identities.