Posle dozhdichka, v chetverg

A palace steward conspires with bandits to kidnap an infant czar's son and an orphan adopted by the czar, replacing the prince with the steward's son. Two decades later the true prince and his foster brother are slaves in a quarry, the changeling grew up as a prince, and his mother has been elevated to the highest position in the state for "saving" the prince, but losing her own child. Then the brothers escape, the changeling prince departs to fight an immortal wizard and vanishes, and the brothers have to deal with the wizard. Which includes stealing a bird-woman from a Mid-Eastern khan and figuring where the wizard's death is.