Sozin's Comet finally arrived at the end of the summer as predicted. It increased the Firebenders' power by a hundred times, giving them the power required to exterminate the Fire Nation's enemies. Fire Lord Ozai planned to use these enhanced powers and his personal army of Airships to burn the Earth Kingdom to the ground. In response to this plan, Avatar Aang and his allies launched their attack, with Aang facing the Fire Lord while his friends attacked Princess Azula and the Fire Nation invaders.

Phoenix King Ozai burning the Earth Kingdom with enhanced Firebending
During the battle, the power of Aang and his allied firebenders' powers also increased, giving them an advantage on the battlefield as well. Masters Jeong Jeong and Iroh of the Order of the White Lotus used these powers to liberate Ba Sing Se. Zuko used them to battle with his sister Azula, whose mental instability hindered her battlefield prowess, but also made her all the more dangerous.
Finally, Aang used these powers combined with his skill in the other elements to battle Fire Lord Ozai, whose skills with firebending were superior to his own, on a near level footing. However it was shown that even with the power boost of the comet, master benders of the other elements, most notably Aang, Bumi, Pakku and Katara, were not completely outclassed. Despite their enhanced strength, Ozai and his allies lost to the Avatar and his group. Azula was finally defeated and Ozai's armies were forced to retreat. Aang used Energybending to strip Ozai of his firebending, rendering him powerless.
By the time Sozin's Comet disappeared into space, the Earth Kingdom was liberated and the War was finally over. Aang had fulfilled his destiny to save the world from the tyranny of the Fire Nation.