In Twilight Princess, Zant is the one who rules the Twilight. He is the main antagonist of the game and possesses powers beyond Link's comprehension. He is also a member of the Twili race. There are multiple ways of defeating Zant, the first suspected way being the three Fused Shadows. When Zant steals these away from Midna and returns her to the World of Light, the only option left to defeat Zant is to find the Twilight Mirror.
Zant self-proclaims himself "Twilight King," with the ultimate goal in mind of spreading the Twilight across Hyrule to make way for the Twili to return to the world. His position is not rightful, and he is soon revealed to be a usurper, having stolen the throne from Midna, the rightful twilight princess. Presumably he was to be the next in line for the throne, but was skipped over and his position was filled by Midna instead. This led him to lose trust in the Royal Family of the Twilight and seek his own path. He prayed outside of the castle for greater power in order to claim his place on throne.
Ganon became his answer. Ganon's evil spirit, entrapped in the Twilight Realm, approaches Zant and shares his power with him, more or less only fueling his own desire to return to the World of Light. After Zant is defeated, he proclaims that he cannot be truly killed until his god, who is in reality simply Ganondorf, is also destroyed. Midna, hearing this speech, destroys him with a mere fraction of the power of the Fused Shadows.
At the very end of the game, when Ganondorf is dying, a vision from the sky of Zant's neck snapping along with Ganon's, symbolizing the true death of both villains.
Boss Strategy
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Zant is one of the more complicated bosses out of all of them. He will at first teleport both himself and Link to the Forest Temple boss arena and float above the poisonous water and shoot you with balls of dark energy. Block these with your shield and use your Gale Boomerang to knock him down while he tries to get more energy. He will bounce around and land on the ground, attack him with your sword. Repeat this a few times and he will be defeated in this area.
Next he moves to the Goron Mines boss arena and continuously will keep jumping making the platform move down. Avoid falling down by using the Iron Boots when necessary. Eventually he will stop and attempt to attack you again. When he is tired remove the Iron Boots and quickly attack him. Repeat this a few times and he will be defeated in this area as well.
Next he moves to the Lakebed Temple arena. Be sure to equip your Zora Armor to breathe and your Iron Boots to float to the ground. There will be multiple helmets like Zant's on the ground. One will open up and inside will be Zant and he will attack you. After he is done he will wait a few seconds before he closes the helmet. At this point quickly use the Clawshot to pull him out and attack him. He will go back into the helmet and you will have to figure out which one he is in and then repeat the process.
Next he goes to the mini-boss arena in the Forest Temple. He will jump around the pillars and first use your Clawshot to stun him, and then roll into the pole to knock him down, and attack. Repeat this a few times and again the arena will change.
Next he goes to the Arbiter's Grounds arena and you will have to attack him the same way you did with Stallord's head.
After attacking him a few times in Arbiter's Grounds, he will go to the Snowpeak Ruins and jump around the area in a giant form. Hit him in the feet with the Ball and Chain to make him shrink down into a size that you can attack him with the sword. Repeat this a few times to go to the next area.
The final arena is just outside of Hyrule Castle. He will simply attack you with two swords he has and you have to avoid them. Occasinally he will do a spin attack and be left dazed for a few seconds. This is the best time to attack. After a bunch of hits, Zant will be finally defeated.