usurper
Refers to a person who seizes power, position, or rights illegally or by force. Often used in political or historical contexts.
彼は簒奪者として王位を奪った。
He seized the throne as a usurper.
Compound of 簒奪 (usurpation) + 者 (person). The exact historical derivation is uncertain, but the kanji spelling is conventionally associated with the meaning of usurpation.