Someone on tumblr did some digging around with some photoshop and found that x rayish pic is from the scene when Zaheer announces the earth queens death to baa sing se with ming hua and p'li at his sides
An army of "peacemakers" from the spirit world that have come to instill order in those that have escaped to cause havoc on the material plane. However, with the spiritual and physical maiming of the Avatar, they decide that it's high time they took over for the good of all as both spirits and humans seem incapable of taking care of themselves.
Spirit-Human relations are a big part of the book. Neither side is laudable as Spirits are usually full of themselves to the point of conceit while Humans can be casually cruel. Last half of the season is basically both camps trying to overcome their differences and find a way to defend themselves against this powerful army that would judge them all.