When he was a child, a very young boy, his Father had ripped him out of his mother's arms and taught him the sting of the flame. Even at that age, he had recognized the meaning of the spark within his being, that warm ember deep inside that was never extinguished even as he slept - and he had pushed back with his entire being.
He did not know fear for a second, even though the eyes of his opponent revealed no mercy.
True, the Firelord had simply batted away his retaliation with a casual wave of his hand. But the pride in his eyes stoked that flame within. Ozai walked with a stinging burn on the back of his right hand for weeks; he never ran back to his mother for comfort ever again. He didn't need to, when the flame inside was all he could ask for.
The Phoenix King was soaring.
The Phoenix King was falling, and for the first time in his life, he understood fear. He saw the power the god-child wielded, and glimpsed his mortality.
The Phoenix King was falling, as the flame burnt white hot within him, before becoming doused in this foreign power invading him.
Then he was drooling on the ground, the jeers mere children around him ringing in his ears.
And the Avatar called this mercy.
The Avatar calls this mercy, he thinks, staring up at the bars that have become his sky.
Down the hall, he could hear his daughter's blubbering and sobbing, screeching at unseen phantoms. He knows what the guards do to her in the dark of the night, knows from the change in her screams. He wonders if they have bound her hands permanently, because a mad Lightning Goddess was still a Goddess - so long as she had use of her hands. He wonders if she still has hands.
He's not sure he cares; he thinks, idly how he had never seen that madness tinging her eyes before everything had turned to cold, greasy ash.
He thinks sometimes of his son who hates him so deeply, never understanding that yes, his love for his Zuko had been conditional and laced with disappointment, but it was still love. He wonders if what he feels is pride when he sees Zuko striding in, proud and tall, every few months in full Fire Lord regalia, trying to extract all his secrets from him.
Mostly, he lies there on the ground between eating, between fits of sleeping, wondering how it was possible for him to be so cold. The fire he had known all his life was gone, and he couldn't find it again, no matter how deep he looked.
The Avatar calls this mercy.