Usurper

The "Usurper" was an aristocrat who, a few years before 1237, overthrew the Nilfgaardian Empire's Imperator, Fergus var Emreis, then seized the title for himself. He ruled until he was overthrown and summarily executed by Emhyr var Emreis' supporters in 1257.

After assuming the throne he granted nationwide amnesty, excluding the political prisoners. He also continued the conquests of neighboring lands in the north.

Accession
In the 1230s, an opposition rose to prominence with the sole purpose of dethroning the House of Emreis with a certain nobleman leading it. When he gathered enough strength, he led a successful revolt against the Imperator and usurped the throne of Nilfgaard for himself.

After the coup, he tortured Fergus in an attempt to force him to acknowledge the usurper's rights to the throne. Fergus resisted and was ultimately killed. Fergus' son Emhyr, who was thirteen at the time, was cursed by the mage Braathens into a humanoid hedgehog form, since Emhyr actually means hedgehog in the Nilfgaardian dialect, and banished from the capital of the Empire. Usurper presumed that he would be killed by hunters but Emhyr managed to hide with the help of Ardal aep Dahy and then fled beyond the borders.

Reign
Shortly after he stabilized the situation at court and therefore the Empire, he proclaimed overall amnesty concerning everyone sentenced by his predecessor, excluding only those "guilty of high treason" like Vysogota of Corvo.

In 1239, Imperial troops annexed Ebbing and began what would be dubbed the Northern Wars. He continued this conquest by military onslaughts and political maneuvers, taking control of Metinna and Nazair. In the 1250s, Maecht was also conquered.

At some point during his reign, he diminished the cult of the Great Sun's rights.

Downfall and death
In 1257, an uprising led by Ardal aep Dahy, Vilgefortz, and Emhyr's other supporters broke out against the Usurper. They managed to rally several soldiers and cadets and, shortly after Emhyr himself reappeared, stormed the imperial palace, overthrowing and executing the Usurper in the process. Following all the power struggles around the throne, Emhyr had all of his dead political enemies disinterred and used their gravestones to pave his ballroom, earning him the nickname "the White Flame Dancing on the Barrows of his Enemies" or in Elder Speech, Deithwen Addan yn Carn aep Morvudd.

Trivia

 * His name is never mentioned in the Saga or games. Curiously, in Wiedźmin: Gra Wyobraźni he is referred to as "Torres" - it is not known if it's authors' mistake whether he had indeed carried a name of one of the former Nilfgaardian emperors.

Notes & references
Uzurpator Узурпатор Узурпатор