Zant
Zant is the ruler of the Twilight, which engulfs much of Hyrule in Twilight Princess. 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 that belongs to them. 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, which, added onto the fact that he hated that the Twili were kept in the Twilight Realm like a caged animal, made him seek his own path. In throes of rage and despair, he looked towards the heavens for an answer.
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 will be resurrected infinitely by his god, who is in reality simply Ganondorf and that the course on Midna cannot be removed. Midna, hearing this speech, destroys him with a mere fraction of the power of the Fused Shadow.
At the end of the game, a vision of Zant's neck breaking directly proceed's Ganondorf's death. One may speculate that the magic of twilight leaving Ganondorf's body along with the Triforce of Power stripped him of his power, and he was killed by the Master Sword.
Boss Strategy
Zant is one of the hardest bosses in all the game, with the ability to teleport to previous battles and change assault tactics.
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 mini-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 sink 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.
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, and Zant will now unleash two swords from his hands. With his two swords he has, he will simply start attacking you like a maniac, which you can either dodge this attack or Back Slice him. Occasionally 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.
Miscellaneous
Zant bears an uncanny resemblance to Onox and Veran look on his clothes, and he has the same symbol Onox had on his armor. Given that Twilight Princess is full of references and close similarities to Oracle of Seasons, and Oracle of Ages, this is probably not purely by coincidence.
Not only does Zant's appearance resemble that of General Onox and Veran, but the Twili textures, shapes, and patterns can also show a similar cultural background to those who created Majora's Mask. However, a resemblance of clothing is not the only thing that reveals itself when analyzing Zant. His combat techniques, when facing him head-on, are similar to the techniques used by the several forms of Majora's Mask, Ganon, and Phantom Ganon. He brandishes his blades in a fashion that is just as devastating as the Majora's Wrath whips, teleports with a manner of cunning attacks, and uses consistent energy strikes in order to test Link's patience, just as Ganon and Phantom Ganon do.