Karzahni was a prototype plant created by Makuta Teridax and named after the legendary tyrant. The Makuta of Metru Nui intended Karzahni to have a limited lifetime so that it would not kill him.
Karzahni wanted to get Energized Protodermis so that he could change himself to be more powerful and defeat Teridax. The Toa Metru got him the liquid, but it killed him.
Karzahni was used as a flotation device for the Lhikan II. It grew again, and played the role of the Morbuzakh in one of Teridax's illusions. It later told Vakama the truth about who was destined to be the Toa Metru: Mata Nui changed the stars to fool Teridax. The Karzahni plant received this information from the Order of Mata Nui member he tortured long ago.