At the San Diego Comic Con, we saw pictures of a new Spirit Monster who was believed to be one of the main antagonists in Book 2. The fan base was mostly in the dark in regards to the new Spirit Monster until Grey DeLisle decided to shed some light on the character through her twitter.
First of all, in case you may have never even known, Grey DeLisle is best known to the fandom for being the voice actress behind the infamous Azula. And her tweet regarding the Spirit Monster was quite interesting.
I will be voicing a character on The Legend of Korra called "The Dark Spirit"!!! No word on the return of Azula...YET;) *Evil Laugh*—
Grey DeLisle (@GreyDeLisle) July 17, 2012
So it looks like Grey DeLisle will be a part of the Avatar Cast once again to voice The Dark Spirit.