Karzahni has direct control over his realm and immense power inside it. He battled his brother, Artakha, for the Mask of Creation, which he lost, and was instead gifted with the mask of possible futures, the Olisi. Constant usage of this mask to try and determine the future led to many inane, horrific possible end results for the universe, which began eating away at his capability to perform, directly affecting his work of repairing damaged Matoran, and his fragile mind began eating away at the laws of physics in his realm, sometimes flipping things on their heads, with sand waterfalls and tiny ripples in water equivalent to sonic booms.
Karzahni used the broken state of his realm to keep Matoran who should have been dead long ago still alive and deteriorating, toiling away endlessly as he waits for more Matoran to toy with. When the Inikatoran (Jaller, Hahli, Kongu, Matoro, Nuparu, and Hewkii) arrived in pursuit of the Toa Nuva, Karzahni imprisoned them and gave them masks of power to wear, although Hewkii wore one he had snatched along the way. Later on, the six Matoran attempted to flee the island in Toa canisters, but were confronted by Karzahni, who was manipulated by Matoro into revealing to himself that Mata Nui - an unimaginably more powerful being than Karzahni with control over the whole universe - existed, baffling him long enough for them to escape.
Later on, Karzahni's realm was destroyed by Gali's use of a Nova Blast, which shredded the place. Karzahni fled to Mahri Nui, where he was mutated by the water there, and eventually confronted Teridax in the form of Maxilos. He used his mask to give Teridax a nightmarish alternate future of Mata Nui undoing his master plan startling enough to shock the leader of the Brotherhood of Makuta into a scream. Teridax repaid this act by launching Karzahni out of sight.
Karzahni would go on to torment Toa Lesovikk, a Toa there specifically to kill him, with his mask powers to force him into a reality where his Toa team never perished. Toa Lesovikk would later see Karzahni trapped by two Matoran using an elaborate cage, and given the opportunity to kill him he would choose to let him suffer with being alive instead.
Karzahni would later perish before Teridax was killed at the hands of Mata Nui.
That's a general synopsis to give you the basics of the character. Anything more specific would probably warrant a different topic.