The Avatar, Alchemists and Alkahestry (sequel, number 4 in series)

Edward and Ayana (OC Avatar) travel back to Amestris to visit their friends and wind up in Xing for Ling's coronation. But they make a horrific discovery about the country's history, and it sets them on a journey over the vast landscapes of Xing as they try to set past deeds right again. Meanwhile, a grudge-holding prince aims to satisfy his thirst for revenge.