Avatar: The Legend of Aang, Book 4: Time

A threat coming from the future comes to give Aang and his friends a new challenge but, how difficult it will be? Let's say that worse than Ozai itself, she is someone that won't stop at anything to fulfill her mission, eliminate the avatar and the fire lord of that time and conquer the 4 nations, will it make it, or the Gaang will win once more? (Warnings on first chapter)