usurper

Concise Oxford English Dictionary © 2008 Oxford University Press:
usurp /jʊˈzəːp, jʊˈsəːp/

▶verb

• 1 take (a position of power) illegally or by force.

  ■ supplant (someone in power).

• 2 (usurp on/upon) archaic infringe on.

– derivatives
usurpation /ˌjuːzəˈpeɪʃ(ə)n, ˌjuːs-/ noun,
usurper noun.

– origin ME: from OFr. usurper, from L. usurpare ‘seize for use’.