usurper

noun

1. One who usurps.

   “The fact that the Yongle emperor was therefore a usurper, regicide and nepoticide (nephew-killer) made compiling the Veritable Record – or official history – of his reign a most dangerous scholarly post. The official given this ticklish task managed to survive several drafts, finally producing one that pleased his master as it omitted the dead nephew's reign altogether.”