Usurper

(n.) 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.

Thesaurus Entries

Simon Legree, absolute monarch, absolute ruler, all-powerful ruler, arrogator, autarch, autocrat, caesar, commissar, czar, despot, dictator, disciplinarian, driver, duce, hard master, martinet, oligarch, oppressor, pharaoh, pretender, slave driver, stickler, tyrant, warlord