To avoid latecomer spoilers, this troper will instead list reasons WHY "Embers" is so mind-blowingly awesome
Vathara doesn't write flat characters, period. From the main cast to the villains, right down to the spear holder roles, Vathara gives every character a real, human feel. Flanderization? Not in Embers.
Hand Waves, Fridge Logic, A Wizard Did It, and every "Rule Of" trope have no place in this fic, and Vathara does her best (which is pretty darn impressive) to explain canon fridge logic, point out unfortunate implications, and make sure everything is not just plausible, but believable. A lot of world-building gets accomplished this way.
All actions actually have consequences. Even if it means disastrous results, nothing gets left as a loose end. (ESPECIALLY if it means disastrous results!) Hero Insurance? Status Quo Is God? Both have been munched by Vathara's thorough Deconstruction.
Gambit pileup is prominent here. While the canon plot of ATLA gives the protagonist (Aang) a task (defeat the fire lord by the end of summer) that ends the series when completed, the cast of Embers finds the gambit pileup to be higher and higher the farther in they go. Defeat the fire lord? Sure. The REAL problem is WAY bigger in scope, complexity and overall gravitas; and Vathara doesn't do the cast any favors by leaving a Trail of Breadcrumbs. The crumbs they DO find aren't even in the Convenient Correct Order.
Vathara has to be the most extreme case of Writer Did Her Research that this troper has ever seen. She applies an extensive knowledge of psychology, biology, mythology, real world history and an absurd number of other studies to Embers. She will often have a list of recommended reading in her author's notes that she herself has read, and used while writing the fic.
To wrap this all up, Vathara's "Embers" is this troper's favorite work of fiction. This includes ALL works of fiction, published or otherwise, and yes; including the canon ATLA series. This troper holds Embers in such high regard that an update for Embers holds significance on par with a religious holiday. Make sure that your schedule is clear when sitting down to read it for the first time. It's a powerfully addicting read.