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.

Usurper is a BBS door game created by Jakob Dangarden in 1993. The interface and gameplay are similar to 1989's Legend of the Red Dragon. The most recently released version was 0.20d.

One who has successfully completed a Coup D'etat.

an individual who successfully lays claim to what rightfully belongs to another