The phrase "imperial throne" refers to a seat of power or authority in an empire. It represents the ruling position or seat from where an emperor governs. Full definition
He was born four years after Kaiser Wilhelm II ascended the German imperial throne; he died nearly a century later, in the same decade that witnessed the fall of the Berlin Wall and the reunification of Germany. (firstthings.com)
The fourth Crusade, called by Innocent III, stormed Constantinople (1204)-- although that had not been the Pope's wish — placed one of its leaders on the imperial throne, and seemed to promise the reunion under the Pope of the Western and Eastern wings of the Catholic Church. (religion-online.org)
Glyphs on the codex indicate the tribute paid to the Aztecs by conquered towns just before the Spanish conquest; the towns on one tribute list (in what is now San Luis Potosí) gave 1,600 loads of dry chile to the imperial throne each year! (fieryfoodscentral.com)