| I've decided to focus on trying to explain the Dark Avatar's origins (there are a few versions), shadow and lightbending, and a few odds and ends. I.) The Dark Avatar's Origin A.) Origin 1: Evil Twin Scenario While cliche, the 'evil twin' scenario has its merits. The basic concept is that the Avatar Spirit was intended to be the perfect personification of the world. But humans are far from perfect, and as such the human spirit introduced the concept of good and evil to the world. The Spirit of the World (For the sake of naming the mother of the Avatar Spirit, let's call it Gaia), in order to maintain it's original vision, seperated the good and evil aspects of into a pair of identical twins, each having a seed of the other's essence to maintain the balance, Yin (Evil/Dark), and Yang (Good/Light). As these two grew, Yin became ostracized from the human commnuity because he was more prone to 'evil actions' than his brother, Yang, was. As such he fully embraced his dark side and became the Dark Avatar, dedicated to destryoing the world that rejected to him. This hatred was passed down to his descendants and incarnations, marking early history with periods of destruction, chaos, and despair. B.) Origin 2: The Fallen Avatar Scenario Somewhat more likely, but more like a rip off of Star Wars, this scenario involves an incarnation that fell from his/her duties and abused his/her power to the point that faith in the Avatar was nearly destroyed. In order to save their representative, this incarnation was forcibly removed from the Avatar line, seperated from it's past and future lives to prevent corruption. C.) Origin 3: The 'Darkness Within ' Scenario We have seen this in the canon series when Aang demanded to know where Appa was taken in "The Desert". The distortion of the voice overlay had a very dark overtone to it. This scenario seems to be the most likely in part where Aang has an internal battle with a Dark Avatar. II.) Shadowbending and Lightbending In order for the series to remain fresh, you need to have new material to work with. After season three, the only thing left for the Avatar to do is to help rebuild the nations. Naturally, you need escalation in order to keep a series going, and in order to have an effective escalation, you need someone who can provide a significant challenge to your main protagonist. If you have a protaganist who is supposedly the only person who can manipulate all four elements, how do you challenge that? You either have someone who can also manipulate all four elements at equal or greater skill, or you have a new power that cannot be immediately handled. Therefore, I created shadow and lightbending, which are in fact the same element, aether, just of different types of Chi, positive, and negative. In order to up the ante, so to speak, the Dark Avatar would have to have a unique form of bending to set itself above other benders and Aang himself initially, hence Shadowbending. I believe the real world martial art this could be based on is Tien Shan Pai, the description of which can be found at this link on wikipedia.org. As for Aang, he should learn the opposite form of bending which does not out****Shadowbending, but merely cancels it out, Lightbending. Lightbending could be based on the real world martial art of WingTsun, the description of which can be found at this link, from wikipedia.org What makes these possible is that I believe the Yin and Yang of negative and postive Chi is the true source of the bending disciplines, the crossroads, if you will. By standing in this crossroad, Aang can manipulate the 4 elements. Why can't somone manipulate the source itself, what would that look like? I figure it could like this, note that the yin yang symbol move in the same clockwise direction as the Avatar cycle: 
III.) Who should be the Dark Avatar? A.) Princess Azula I feel Azula would make the best candidate because her perfectionist nature would allow her to learn the four elements (Air, Water, Earth, and Shadow) very rapidly, perhaps even faster than Aang did. This reduces the time gap between seasons 3 and 4 and allows for the nations to become complacent and ripe for a new conflict. B.) 'Dark' Aang A Dark Aang would make for an interesting internal battle, but wouldn't have much dramatic impact on the world for a whole story arc.This also smacks of Anakin/Vader. C.) Unknown Character Perhaps the Dark Avatar is already here, hiding behind the scenes, his/her fingers moving the pieces into position for 100 years. | 
