Embers is...well, it's certainly an interesting story. It started out life as an Avatar: The Last Airbender
fanfic, but by now has diverged significantly in tone from its founding source. If you like elaborate world-building, rules about spirits, the development of culture when concepts such as justice and family are a spiritually binding force on the people, and discussions of morality, it's well worth a go.
That said, the characterisation is polarizing — some people love it, believing that the characters are deepened by Vathara's use of real-world psychology in the same way the setting is improved by her use of real-world myths and geography. Others, however, find that her interpretation of the characters clashes with the sense of them they get from the show, and disagree with Vathara's take on morality and responsibility, which is a great deal harsher than the show's. Fans of Aang and Katara may want to give this one a miss; Vathara writes very well, but the flip side of that is that when she dislikes a character, it hits home hard.