Karzahni was a plant created by Makuta Teridax and named after the legendary tyrant. Teridax intended Karzahni to have a limited lifetime so that it would not kill him.
History
The Karzahni was created by Makuta Teridax as a prototype for the Morbuzakh long ago. It was created to help Teridax in his plan to enslave the Matoran in Metru Nui. Teridax wanted Karzahni to stop the Matoran from going to the coast of Metru Nui, so that they could not escape to other lands. However, Karzahni was too intelligent for Teridax and did not want to be part of his plan. Because of this, Teridax abandoned Karzahni, and later created a less-powerful plant and named him the Morbuzakh.
During its time in his tunnel, it tortured an Order of Mata Nui member into giving away that they were altering things to make Teridax think that Nuhrii, Ahkmou, Tehutii, Vhisola, Orkahm, and Ehrye were to become the Toa Metru. Karzahni wanted to get Energized Protodermis so that he could change himself to be more powerful and defeat Teridax. When the Toa Metru encountered it, they made a deal with him; if he healed Nokama's wounds they will give him the protodermis. However, instead of transforming him as he thought it would, it killed him.
Parts of Karzahni were then used as a flotation device for the Lhikan II. Using the liquid protodermis of the Silver Sea, it regenerated its body while retaining the memories and returned to Metru Nui.
Teridax encountered it and it helped the Makuta by playing the role of the Morbuzakh in one of Teridax's illusions. During the illusion, it emerged from the ground in Po-Metru and grabbed Vakama, but was stopped by a Visorak Boggarak, whom Vakama believed was Matoran Nokama.
Later, after his Kanoka launcher was destroyed, it saved Vakama from death by catching him while the Toa was falling out of the sky and dragged him underground. It then told Vakama the truth about who was destined to be the Toa Metru: Mata Nui changed the stars to fool Teridax into thinking six others were destined to become Toa. The Karzahni plant received this information from the Order of Mata Nui member he tortured long ago. Teridax then arrived, bursting through the ground and killing the Karzahni again.
Powers and Abilities
The Karzahni had the power to regenerate itself while immersed in liquid. It also had sentience, allowing it to speak and think. Unlike the Morbuzakh, it didn't need to feed on Fire and was not weakened by Ice.