Soul of Fire
He first came into being more than a century ago.
True, in some sense he had existed long before that- existed for as long as the will to quarrel amongst themselves had existed among mortals, which was to say from the beginning. But when Fire Lord Sozin had given the order that the Air Nomads were to be destroyed in order to prevent the rise of a new Avatar, the world had shifted permanently. And because the Spirit World and the mortal world reflected one another, that shift had sent shockwaves across them both. In the aftermath of that cataclysm he arose- Zhan Zheng, the spirit of war.
For a century he hung on the fringes of the two worlds, basking in the chaos and destruction that fuelled his essence. But then the unthinkable happened- the Avatar, incarnate guardian of the balance and thus his natural enemy- returned from obscurity and vanquished Fire Lord Ozai utterly. To mortals those events took months, but from a spirit's timeless perception they happened in an eyeblink. For the first time in his existence, Zhan Zheng knew fear.
Spirits embodied the powers at work in the mortal world, and even as the Moon Spirit might be crippled by the loss of the celestial body to which she was bound, or a nature spirit broken by the devastation of its territory, the advent of peace threatened an ending to the incarnation of War. He knew that in order to preserve his own existence and power he had to act- and thankfully, the foolish mortals seemed all to eager to assist him.
He ensnared a delusional Fire Nation general named Azun, convincing him with visions that he was Agni, patron spirit of the firebenders. Azun would have begun the war anew, but he was foiled by the Avatar, and by another, a mortal Zhan Zheng had instructed him to seek out, the one who was closer to the spirit's heart than any other mortal alive. He tried to turn her to his cause and failed, but he was patient, and knew that her time would come.
During the last days of the War, he had claimed the soul of a girl in Ba Sing Se who had been made into a hollowed out puppet for the city's controlling power, the Dai Li. She was filled with hate and rage, and after Azun's failure he decided that through her he could force the world back towards war. She led him to Long Feng, head of the Dai Li, but the earthbender proved too cold-blooded to make an effective pawn. Zhan Zheng was not sad when the girl Wei Ming killed him. Though it had been Long Feng who had most directly ruined her life, some of Zhan Zheng's interest for another mortal bled over into her, and she became obsessed with destroying her.
Again the fascinating mortal proved triumphant, driving the spirit from his pawn's body with searing lightning. But she then provided him with the key to victory- an ambitious warlord hovering on the edge between life and death. Zhan Zheng could only communicate with certain kinds of mortals, normally- the spiritually sensitive, the insane, and the dying who stood on the threshold between worlds. It was easy to trick the warlord into allowing him to possess his body in return for healing.
Like all spirits, Zhan Zheng possessed great power, and he could use it to enhance the abilities of his mortal pawns. To Wei Ming, come from the dark pits of Lake Laogai, he was able to bestow the ability to call shadows and disappear, in addition to turning an untrained peasant into one of the deadliest warriors alive. With the warlord Jian Chin the effects were much more dramatic, his already potent combat skills and earthbending enhanced a hundredfold, and his brutish mind aided by a far greater intellect.
To travel to Ba Sing Se, capital of the largest nation in the world, and shatter it was the spirit's goal now, one that Jian Chin in his foolish drive for glory proved most pliable. But there remained two faces that haunted Zhan Zheng's mind, mortals that could not be ignored. One was the Avatar, vessel of his immortal enemy. The other was a girl who was powerful, cunning, and ruthless, born of the bloodline to which he owed his very existence. So closely did she align with his own desires that she might have been made for him. In the end, he vowed, she would be made to come to him, and he would cast off Jian Chin like the refuse that he was and claim her as his link with the mortal world.
And if she refused, then Princess Azula, like the Avatar and all others who thought they could build a lasting peace among mortals, would soon learn the utter magnitude of her folly.