- The ancient ruler Karzahni; legends state that poor workers were sent to him to be punished.
- The Realm where the aforementioned ruler reigns.
- A plant monster created to terrorize the city of Metru Nui. It was named "Karzahni" after the legends of the ruler.
Most Matoran only know of the ruler and his realm through legends (the plant is unknown for the most part, though the Matoran of Metru Nui know of it through their Turagas' stories). Given the widespread nature of these legends, Karzahni is a common figure in exaggerated "boogeyman" horror stories. Angry Matoran also occasionally shout at each other to "Go to Karzahni!" (a direct reference of a human expression concerning hell, though possibly a little less profane considering how flippantly the Matoran refer to the ruler).
As Karzahni and his realm became myths, they described the land as a nightmarish world; one shrouded in darkness and even hotter than Metru Nui's Great Furnace. This land had no name, but eventually also became known as "Karzahni".
The Matoran that became the Toa Inika recently rediscovered the dark realm during their travels. The land is indeed as dreadful as the legends said; there are volcanoes that erupt burning ice and waterfalls of dust. In addition, thunder makes no noise, but a breeze of wind can be deafening. The pathways are filled with "statues" of Matoran: lazy Matoran who sat on stones for too long and turned to stone themselves. In the center of the realm is Karzahni's palace, once a magnificent home for a ruler, but now a smoldering wreck, and the forges and furnaces used by the Matoran who live there.
Karzahni has long been watched over by a Makuta named Icarax. Despite the fact that the Makuta were once guardians, Icarax never rescued the Matoran; he felt that workers bad enough to be sent to Karzahni were worthless. With its ruler gone, Icarax sought to conquer Karzahni and establish a power base there. However, the Toa Nuva confronted him (after evacuating the Matoran) and, in desperation, Toa Gali unleashed the full extent of her water power, completely destroying the realm.
When plotting his scheme to take over Metru Nui, Makuta designed an intelligent plant creature to drive the Matoran towards the city's center; he named this creature "Karzahni" after the fabled tyrant. However, the Karzahni was too powerful and too ambitious, so Makuta abandoned it and instead engineered a weaker plant creature, the Morbuzakh. Karzahni swore revenge on Makuta, but this was taken from it when the Toa Metru imprisoned Makuta during the Great Cataclysm.
When the Toa Metru were returning to the city, Toa Nokama was critically injured by the Rahi Nui and Karzahni took the opportunity to strike a deal with the Toa: it would cure Nokama if the Toa would get him some energized protodermis. They did so, and Toa Nuju figured out that the Karzahni not only wanted the protodermis to become more powerful, but to save its life; as Makuta deliberately designs a fatal flaw in all his creations. However, the Karzahni was not destined to be transformed by the protodermis, so it was instead destroyed, leaving nothing but dead wood.
Shortly afterward, the Toa Metru used the dead Karzahni branches as part of a boat to bring them to Metru Nui; but none noticed a single sprig that seemed to have sprouted when immersed in the water. This sprig later grew into a new Karzahni plant, with all the memory and experience of the old. This Karzahni hid for a while, and when the now-free Makuta sought the Vahi Mask of Time, he made a deal with Karzahni and had it impersonate the Morbuzakh in an illusion he crafted for Toa Vakama. But after the illusion had been shattered, Karzahni switched sides and helped Vakama by bolstering his self-confidence, revealing that despite evidence that he and his team had found, they were always the ones destined to be Toa. It also said that it had gotten this information from a member of the Order of Mata Nui (and implied that it killed the member). By this time, Makuta had caught up with the two, and Karzahni was seemingly killed again when Makuta encased it in a shell, cutting it off from heat and light.
The Karzahni had full control over its vines and branches and possessed great strength. It could also produce healing sap and fruits at will. It communicated using telepathy and was able to gather information through any of its growth.