Usurper

A usurper is an illegitimate or controversial claimant to power, often but not always in a monarchy. This may include a person who succeeds in establishing himself as a monarch without inheriting the throne or any other person exercising authority unconstitutionally. A person who takes the power of the country for himself or herself at the expense of the people. It may also be applied to an official acting ultra vires, outside his authority or jurisdiction.

References

Other Languages
azərbaycanca: Uzurpasiya
беларуская: Узурпацыя
български: Узурпатор
čeština: Uzurpátor
español: Usurpador
Frysk: Usurpator
Հայերեն: Ուզուրպացում
hrvatski: Uzurpacija
ქართული: უზურპაცია
latviešu: Uzurpācija
Nederlands: Usurpator
日本語: 皇位簒奪
norsk bokmål: Usurpasjon
română: Uzurpator
русский: Узурпация
slovenčina: Uzurpácia (moc)
slovenščina: Uzurpator
srpskohrvatski / српскохрватски: Uzurpacija
svenska: Usurpator
Türkçe: Gasp (siyaset)
українська: Узурпація
中文: 篡位