Bitterbynde
The Bitterbynde is a fantasy trilogy by Australian writer Cecilia Dart-Thornton. It comprises The Ill-Made Mute, The Lady of the Sorrows, and The Battle of Evernight.
In the first book of the trilogy, the story is about a mute with no memory, eventually named Imrhien, whose face is deformed due to a poisonous plant. The mute is trying to find a cure against the poisoning, a name and lost memory. The mute later meets the warrior Thorn who captures her heart.
In the second book of the trilogy, the mute, now having found a cure, able to talk and known as Rohain, lives in the royal court. She later finds her heart's desire, the warrior Thorn, who is actually the King-Emperor James the Sixteenth. The King-Emperor is the Supreme Ruler of the Empire of (The Known Lands of) Erith, which dominates the world, Aia (or as some believe, the southern half. Others think Aia is a half orb because of a barrier called the Ringstorm at the equator). Rohain also finds her memory at the end of the book.
In the third and final book of the Trilogy, the protagonist, first called Tahquil and later by her real name, Ashalind, sets out to find the gate to a lost "Fair Realm" called Faerie, on the northern half of the world, Aia. However, she is deterred and is taken prisoner by the main antagonist, a Faeren rebel and crown prince named Morragan. It's during this time that Ashalind finds out that James the Sixteenth is dead and that Thorn is actually Angaver, the High King of Faerie and the Faeren, and brother of Morragan, to whom James gave the Empire to until the Prince is old enough to rule. Ashalind is later freed and Morragan killed by Angaver. The Bitterbynde Trilogy ends with Angaver and Ashalinnd going to be wed in Faerie, but being separated by the reincarnated Morragan and one of his followers, Nuckeluvee. seven years later, in the epilogue, Angaver is reunited with Ashalind, who loses her memory again because of a bitterbynde, and the two leave Erith forever as an eagle (Angaver) and a seabird called an elindor (Ashalind).
A 'Bitterbynde' refers to an unbreakable promise or oath.