Question: how did emhyr become ciri's father?
Answer: Emperor Emhyr var Emreis Prior to becoming emperor, Emhyr var Emreis was cursed and turned into a monster. His father, the emperor, was killed, and Emhyr var Emreis traveled for a time under an assumed name, fathering Ciri, before eventually freeing himself from the curse and returning to Nilfgaard to take the throne.