Usurper

A usurper is an illegitimate or controversial claimant to power, often but not always in a monarchy. In other words, one who takes the power of a country, city, or established region for oneself, without any formal or legal right to claim it as one's own. Usurpers can rise to power in a region by often unexpected physical force such as via a coup d'état, as well as through political influence and deceit.