Ozai feigned shock.
"What? How?"
"I found out about what he told you, his order to kill Zuko, and I came to talk him out of it. I told him it wasn't necessary to sacrifice Zuko, that it was wrong to make an innocent child an object of punishment. He insisted it was the only way. He began to leave, and I grabbed his robe to stop him. I guess he slipped... and then he hit his head on the floor... Ozai, what have I done?" Ursa sobbed, clutching her husband's sleeves. She may as well have had "scapegoat" tattooed on her forehead.
"Ursa... there's only one way to resolve this," Ozai said solemnly. "You must leave. Leave, or the Sages will have you put to death for this. I can make it look like an accident, but you must get out of here. Go into hiding, and I will send for you when it's safe to return." Tears rolled silently down Ursa's cheeks as she nodded and hurried to her room to gather her things.
"What do we do now, Dad?" Azula asked once Ursa was out of earshot. Wordlessly, the prince proceeded into the throne room. His father still lay on the cold tile, not moving. Kneeling beside him, Ozai checked for a pulse. The Fire Lord was, in fact, still alive. But not for long. He turned to Azula, who handed him the dagger she had lifted from Zuko's belt earlier in the day. Ozai held his father close, then buried the knife between Azulon's shoulder blades. Azulon gasped, then exhaled softly as the life drained from his body in a trail of blood that seeped into his robes and disappeared in the red fabric. Ozai let the Fire Lord's head drop to the tile with a muffled CRACK, his blood pooling underneath his head in a halo of death, completing the appearance that he had simply slipped and split his head open. A tragic, and accidental, death.
Handing the knife to Azula, she hurried towards her room to clean it off while Ozai washed his hands, then alerted the Fire Sages of Azulon's passing. He informed them of the meeting he had with Azulon earlier that day, his request to be named successor, and Azulon's agreement to the proposition. He told the Sages that Azulon was on his way to amend his written will when he had his unfortunate accident. Ignorant of all of this, Ursa crept into her son's room to say goodbye.
"Mom?" Zuko murmured groggily.
"Zuko, please, my love, listen to me. Everything I've done, I've done to protect you," she told him, pulling him into an embrace. "Remember this, Zuko. No matter how things may seem to change, never forget who you are!" With that, she turned, pulled her hood over her head, and disappeared down the pitch black hallway.
When Zuko awoke the next morning, his memories of the previous night came rushing back to him. He ran through the palace halls, calling out for his mother. Azula leaned against a pillar, playing with Zuko's knife and smirking at the thought of a plan perfectly executed. Hearing Zuko's footsteps, she hid the knife behind her back.
"Where's Mom?" Zuko asked her, panic rising in his voice.
"No one knows," she replied calmly. "Oh, and last night, grandpa passed away."
"Not funny, Azula," Zuko frowned. "You're sick. And I want my knife back, now!" Zuko grabbed at the knife, but Azula jumped out of the way.
"Who's going to make me? Mom?" she asked mockingly. Zuko's face fell at the realization that his only ally was gone. Before she could react, Zuko swiped the knife and ran off towards the courtyard. He found his father staring placidly at the turtleduck pond.
"Where is she?" Zuko demanded. Ozai did nothing to acknowledge the request. Zuko lowered his head, realizing he would probably never see his mother again.
At the funeral, Zuko listened reverently to the eulogy. His grandfather had indeed been an accomplished leader, and his funeral was attended in record numbers. He wondered how Iroh would be crowned Fire Lord, having not yet returned from deserting the siege at Ba Sing Se.
"We lay you to rest," the Fire Sage intoned. "As was your dying wish, you are now succeeded by your second son." Zuko jerked his head up. "Hail Fire Lord Ozai!" the Sage concluded, placing the crown in Ozai's topknot. The crowd lowered itself to the ground, and Zuko's eyes darted to Azula, looking for an explanation. Ozai's face had been calm and resolute as ever, but the wicked smile spread across Azula's face told Zuko everything, and yet he understood none of it. Zuko gazed at the crowd anxiously as another usurper took the throne.