Grey DeLisle, the voice actor of Azula
On July 16, 2012 Grey DeLisle surprised Korra and Avatar fans as she tweeted about returning in Book 2 as the voice of The Dark Spirit. More specifically, from her Tweet and other reliable resources, we know that she will be voicing the antagonist of Book 2. Her tweet read  as follows:

You may not be as familiar with DeLisle, but I'm certain you've heard of her character. Grey DeLisle voiced the character of Azula in Avatar: The Last Airbender. Doing a phenomenal job in Avatar, she successfully captivated the audience with her powerful Azula character.

DeLisle will be voicing The Dark Spirit in Book 2
Below are some pictures from the 2012 San Diego Comic Con. As you can see, these are some shots of the Development Art displayed. This specific shot shows multiple views of the Dark Spirit. If her past roles is a representation of what is to come, we can definitely look forward to Book 2 being the best season thus far.