From SA Mafia Wiki
A Usurper is an uncommon Mafia role. The Usurper shares the standard Mafia Win Condition of dominating the Town, and can communicate with them OOT and perform nightkills. However, in addition, the Usurper win condition requires that the Godfather (or whomever is the "leader") is killed, either by Lynch or the Usurper hijacking the nightkill.
A Usurper is a controversial role, as similar to Scum Masons, the Mafia have an assumption of trust among each other, and to have that trust violated by including a third-party role is seen as unethical. This can be mitigated if the Godfather is warned in advance.
A Usurper is not to be confused with a Traitor.