The Second Stone - The Triempery Revelations, #3

Long live the king... if only they can find him. The Kheld King, Stefan Stauberg-Randolph, is dead. He leaves behind a land in chaos. His brother Handurin is rightful heir to once-mighty Essera. only Handurin is missing. Safely ensconced in another reality, he doesn't remember who he is, or why he was sent away, until the day an immortal wizard named Marenthro shows up on his doorstep. Handurin isn't too happy about having his world turned upside down.
But he's even less thrilled about Marenthro's plans for his safe return to Essera-namely by seeking the protection of the one man who was Stefan's most hated enemy: Dorilian Sordaneon.