Comet-Enhanced Agni Kai

The Comet-Enhanced Agni Kai was a highly destructive Agni Kai fought between Prince Zuko and Princess Azula that occurred in Avatar: The Last Airbender. It inflicted significant fire damage to the surrounding areas. The majority of the battle was between Zuko and Azula, but Katara intervened after Zuko was shot by lightning, a blow which had rendered him unable to fight.

The battle held great political significance, as the two challengers were fighting over the right to the position of Fire Lord, and was also strategically important, as Zuko and Katara were able to defeat Princess Azula and gain control of the Fire Nation in concurrence with Aang's defeat of Phoenix King Ozai, the Liberation of Ba Sing Se, and the defeat of the Fire Nation airship fleet.

Also notable were the attitudes displayed by the battle's combatants. Azula had been known to be calm, precise, and deadly, whereas Zuko had been known to have a short temper. The Agni Kai demonstrated how recent events had affected the two siblings, as Azula finally lost her sanity and Zuko managed to gain the upper hand by staying focused and composed.

History
Zuko and Azula had had a long history of enmity. Zuko was unhappy that Ozai deeply favored Azula over him; likewise, Azula was upset that Ursa, their mother, deeply favored Zuko. This childhood competition expanded into extreme distrust and even hatred. They also both have been the first in line to the throne, due to Zuko's three-year exile which resulted in his temporary removal from the line of succession.

Prior to the arrival of Sozin's Comet, Prince Zuko, along with his allies from Team Avatar, except for Aang, as he was on the lion turtle at the time, met with his Uncle Iroh outside Ba Sing Se, where Iroh was preparing to lead the Order of the White Lotus in its mission to liberate the city. Iroh stated that Zuko had to claim his rightful title as Fire Lord, as Ozai and Azula were not fit to hold such a vital position. Iroh himself did not see himself as fit to rule, despite being the legitimate heir prior to Ozai's ascension; he reasoned that the world would see it as a power struggle between two brothers, instead of the world returning to a peaceful state. Zuko had to accept the role in order to ensure peace between the four nations. However, Azula would not give up her throne without a fight. Zuko agreed to face her in one last battle, with Katara to help him. Zuko and Katara immediately departed to the Fire Nation Capital to carry out their mission.

Agni Kai
The Agni Kai occurred on the night of the arrival of Sozin's Comet, which returned to the Earth every one hundred years and greatly enhanced the firebending powers of every firebender across the globe. Zuko and Katara arrived at the Capital City sometime after the comet appeared in the skies, and just as the Fire Sages were about to crown Azula Fire Lord, Zuko stated that he would become Fire Lord, not her. Amused, Azula challenged Zuko to an Agni Kai, the showdown "that was always meant to be". Zuko accepted, much to Katara's shock. Zuko stated that he did not want Katara to fight when the battle started, since he could sense that Azula, following the betrayal of her former friends Mai and Ty Lee, was mentally deteriorating. In his own words, "There's something off about her; I can't explain it, but she's slipping." He reasoned that this would weaken her prowess, allowing Zuko to fight her evenly without the risk of Katara getting hurt. Katara consented and stayed clear of the courtyard.

Zuko and Azula took their respective positions on each side of the coronation square and fired enormous fire blasts against each other while utilizing the power of the comet. Despite her apparent weakness, Azula fought viciously and launched repeated attacks against her brother with blue fire. However, Zuko's firebending had greatly improved despite his expertise with only red and orange flames, and he deflected many attacks while launching a few of his own. As Azula's sanity continued to crumble, she became easily exhausted, and her attacks became less effective. She also displayed evident fear of her brother's attacks, as his firebending had improved remarkably.

Azula attempted to encircle Zuko, increasing her mobility by propulsion with fire jets, while Zuko attempted in vain to hit her. However, he unleashed a fiery spin kick that managed to knock her down. Taunting her, he attempted to coerce his sister into resorting to lightning generation, in order to utilize that power against her. Despite Azula's awareness of Zuko's lightning redirection abilities, as demonstrated when he used it against their father, she succumbed to his goading and prepared such a strike. Zuko, meanwhile, prepared to execute his redirection technique, realizing that success could determine his victory.

However, noticing Katara standing behind her brother, Azula changed her plans at the last minute and directed the lightning bolt at the unprepared and shocked Katara instead, who watched frozen in horror as the attack approached. Zuko noticed what Azula was doing and jumped into the lightning's path. He absorbed the full force of the blast, only successful in redirecting some of the energy into the sky, while the rest damaged his body, badly injuring him.

Katara intervenes
With Zuko out of action, Katara was forced to face a crazed and violent Azula alone, but her attempts to counterattack were rendered useless when Azula deprived her of any large source of water. Katara took cover, searching desperately for water, while the Fire Nation princess taunted the duo and used her firebending to chase the waterbender around the arena, intent on killing her. Finally, Katara found a storm drain filled with water, as well as a set of chains and came up with a plan to defeat Azula. Azula then cornered her and prepared for a final strike. Katara tricked Azula into standing above the drain, and just as Azula prepared to strike Katara down with supposedly another blast of lightning, Katara pulled up the water and froze herself and Azula in ice. Defenseless, Azula watched helplessly as Katara liquefied the ice around herself and chained Azula's hands together. She then chained her to the storm drain and liquefied the ice. She tightened the chains to ensure Azula could not escape, effectively disarming her.

Katara rushed to Zuko's side to heal his wound and was relieved to see that he was all right. He thanked her for saving his life, but Katara said she should have been the one thanking him. When Azula saw Zuko stand up alive, she finally descended into insanity. Azula screamed and started breathing fire in every direction, struggling to break herself free. She then knocked herself onto her side and sobbed uncontrollably. Zuko and Katara watched with shock as Azula lost control of her emotions.