Chapter 36 - what book IV is all about (summary I)

Pha is a 23 year old girl who lost only has 1 yes, Pha was a non bender her whole life until the tree of time incident, a problem happened and 10 spirits merged together all in one (the details of how it happened will be in book IV) and created the one and only bispirit to have fire and water as their elements, because of how 10 spirits were compressed into one, it granted the person who got the spirit, Pha, immense powers, that she will use with her 2 friends to conquer and try to make the whole earth kingdom submit to her, with an ability that was never seen in any bender before. Yin will be fighting her from one side, and a part of the dark spirit Vaatu from the other (details in the chapter). Why is Vaatu present although it hasn't been 10.000 years yet? what is Pha's unique power? and how will she use it to turn the avatar against his own people? find all of this out, in book IV: peace