One who usurps; especially, one who seizes illegally on sovereign power; as, the usurper of a throne, of power, or of the rights of a patron.
one who wrongfully or illegally seizes and holds the place of another
Usurper (lat. usurpare = to seize for use, to use) is a derogatory term used to describe either an illegitimate or controversial claimant to the throne in a monarchy, or a person who succeeds in establishing himself as a monarch without having inherited the throne in the normal way. Some famous examples are Richard III, Miguel of Portugal and Habibullah Ghazi of Afghanistan.