At long last, the day had come. The invasion of the Fire Nation was about to begin. The timing had been carefully planned, the Fire Nation's defenses analyzed, and any unexpected alterations accounted for. The forces had been gathered, and the plan was set.
The invasion was divided into three parts. The first was the last portion of the Earth Kingdom Fleet, backed up by Water Benders from the Northern Water Tribe, led by Master Pakku. Their objective was to visibly approach the bay before breaking through the Gates of Azulan, and then break through before attacking the defenses of the bay itself, drawing the attention of the defensive force with the conventional assault.
The second group included a good portion of the special squad: Earth Benders, Swamp Benders, most of the Kyoshi Warriors, warriors from the Southern Water Tribe, and Fire Bending prisoners freed from the Boiling Rock who'd been willing to work for them. They would ride in specially designed submarines, created by Sokka and Teo's father, under the Gates of Azulan to attack the beach directly, taking out the defenses further in.
The third group included Aang, Katara, Sokka, Zuko, Cadence, Suki, Toph, Mai, Ty Lee, and Jin. When the problem of air supply came up for the submarines, one of the prisoners from the Boiling Rock had suggested using pumps to put the air under pressure. By combining that idea with Aang and Cadence's Air Bending, numerous air tanks were built, tested, and stowed on the submarines, allowing them to carry enough air to make it all the way to the beaches from the Gates without surfacing. A bit of cleverness with valves on Teo's father's part and some cleverness with fabric on Sokka's part had produced breathing masks for the group and even Appa, allowing the group to fly much higher than Appa normally could. They were to come down on the capital city itself while all attention was engaged with the attacks at the bay, doing a steep dive so they wouldn't be detected on approach until it was too late for any defenses left to counterattack.
As the invasion began, the first two stages went off without a hitch. When the fleet approached the gates, they were raised and ignited, at which point the Northern Water Benders moved forward to extinguish the flames and freeze the gates solid. The Earth Nation ships then extended drills - their designs the same as the one Teo's father unwittingly built for the attack on Ba Sing Se's walls, but on a smaller scale - to break through, while the Water Benders held off the smaller boarding boats sent out to counter attack. Meanwhile, the subs went under the Gates, making their way to the beaches.
When the subs breached at the shore, the Earth Tanks began the assault, even as Hue - the eldest Swamp Bender - gathered enough seaweed to shape his plant based battle body. With that force in place, the majority of the troops began a march through the worst of the defenses while the Kyoshi Warriors focused on taking down battlements, and the Swamp and Fire Benders remained to protect the subs, in case an emergency retreat proved necessary.
When it became plain that the defensive forces were fully engaged, Appa began his dive. With all attention on the attack from the sea, none saw the armored Sky Bison's silent descent, and the group was able to attack the Royal Palace unopposed. Unfortunately, that part of the plan did not go as smoothly.
"The palace is empty!" Aang called out as he finished his search. "There's no one in sight!"
"But that doesn't make any sense!" Sokka shouted angrily. "Where would they go? Why would they go?"
"The return of Sozen's Comet is well known in the Fire Nation," Mai spoke up. "The Royal Astronomers have been tracking the heavens since the start of the year to determine the exact day ahead of time. Maybe they knew the eclipse was coming."
"And when Father saw all his reserves being drawn away, he realized another attack was coming!" Zuko growled. "He'll have withdrawn to the emergency bunker until the eclipse passes! I know the way in, but there are several places within he could be holed up. It's a full maze of tunnels inside the volcano behind the city."
As the eclipse had already begun, the group had to split up to search the entire compound for the Fire Lord, though at one point Cadence was forced to leave the group she'd been sent with behind to cross a wide river of lava.
As a result, she was alone when she found the Fire Lord and his guards.
"Cadence?" he asked, shocked to see her. "What are you doing here?"
Cadence swallowed convulsively. "Hello, Uncle..."
When the eclipse ended, Zuko's forces had complete control of the Fire Nation capitol...but Ozai had not been found. As Zuko and the others began trying to come up with a plan to solve it, Cadence returned...with the Crown of the Fire Lord. Off in the distance, a small group of upgraded War Balloons - metal plated and crewed by a hundred Fire Benders each - was seen withdrawing from Fire Nation territory. "He...ran away?" Zuko gaped, stunned.
"All I found...was the crown," Cadence explained. "He left it behind."
"He...abandoned his country?" Zuko marveled, falling back. "I...can hardly believe it..."
"Does...this mean we won?" Sokka asked hopefully.
"...for now," Zuko allowed. "But my father still has a strong force under his command. There's no telling what he's up to. As long as he's still at large...the world can't truly be at peace. The Fire Nation is no longer an enemy of the world...but the war is not yet over..."
Cadence glanced away as everyone turned to plan the next move, biting her lip.
"It's been a long time," Cadence said calmly. "Things have...changed."
Ozai frowned. "I take it it's not a coincidence you arrive as we're being invaded during a solar eclipse?" he asked softly, coldly.
"I'm afraid not," Cadence replied. "Zuko has seen what the war has done to the world, and seeks to end it. Today is the day the Fire Nation loses the war...but he seeks to keep the nation intact. That's why...he's leading this invasion. He intends to challenge you to an Agni Kai to claim the throne." She paused as she saw the look of shock on Ozai's face. "Will you face him?"
Ozai frowned thoughtfully. "...I cannot," he admitted at long last. "If I defeat him in the Agni Kai, those with him will bring me down after I have been weakened. If he doesn't kill me to win, there will always be the doubt that I let him win, and the people will not follow him wholeheartedly. And if he does...a rule begun in blood will not last long, whether because it destroys him or self destructs." He closed his eyes. "Like he once told me before I gave him his scar, this is a battle that cannot be won." He turned to Cadence. "What other option is there, Spirit Creature? You were sent to my brother. You obviously have some idea about what the future holds for my kingdom, what it must hold. What do you advise? How can this battle end without destruction?"
Cadence hesitated. With what had been said, there was one solution...and one she personally found favorable. But it would go so far against Ozai's nature, she didn't know if he could do it. "...run away," she offered finally.
"What?" Ozai demanded angrily, his eyes narrowed. "You expect me to flee from my own son? Abandon my country?"
"If you want the Fire Nation to survive beyond this day, it's the only option," Cadence replied. "And if you leave your crown behind, I can claim I only found it, and no one will pursue you further. The goal of this invasion is to place Zuko as the new Fire Lord. It doesn't have to be done through you."
Ozai frowned thoughtfully as the seconds ticked by towards the end of the eclipse.
Ozai stared back at his former kingdom as the war balloons withdrew from the invasion force. As Cadence told him, no one was in pursuit. Even so, now that his fire had returned, he had begun to question his decision. To flee? To abandon the throne? What had he been thinking?
As the balloons continued their flight, he knew it was too late to reverse the decision. But as the rage - against himself, against his son, against his brother, against Cadence, and against the Avatar - inside him built, he knew: the war was far from over.
With the comet's return, he would destroy his enemies...and take his kingdom back. ...all of it.