The Usurper is a Mafia role that wins if it outlives its allied Mafia Godfather. This Win Condition is in addition to the normal Mafia Win Condition. Thus, the Usurper must simultaneously work toward the ordinary Mafia win while intentionally bussing one of its power roles. If the Godfather outlives the Usurper or if the Godfather lives to endgame, the Usurper loses even if the rest of the Mafia wins.
The wording of the Usurper aspect of the Win Condition can vary somewhat. Some mods will simply require the Godfather to die before the end of the game, regardless of whether the Usurper is alive.
Sometimes the Usurper will inherit the Godfather's abilities (i.e. investigation immunity) upon death.
Use and Power
The Usurper exists to weaken and/or troll the Mafia. Unless there is a flavor-based reason for including a Usurper, they tend to be frowned upon - more so than Condemner, as the usual intent of the Mafia is that they work as a team.
It is open to debate whether a Usurper should inform the Godfather of its existence. As the Godfather wins with the rest of the Mafia after being bussed, it's simple politeness to do so. On the other hand, informing the Godfather of an impending sacrifice may cause the bus to seem staged, allowing a savvy Town to catch the bussing scum.
While Usurper is not at all commonly seen, some players get suspicious if Mafia power roles die to eliminations that seem too easy, so bussing is not quite as easy as it could be.