Usurper

Usurper - "successor" of Fergus var Emreis. When Emhyr was only 13 year, an unnamed nobleman led revolt against the Emperor which was successful and he usurp throne of Nilfgaard. After that he tortured the Emperor,  forced him to acknowledge his right to the throne, but without success which led to killing him. Emhyr was cursed by Braatens to some humanoid form of hedgehog (that because Emhyr actually means it in nilfgaardian dialect) and banished from the capital of Empire. After some time Emhyr (uncursed) make his way back to City of Golden Towers and overthrow Usurper from throne (with help of Vilgefortz).