The condition which causes a win, almost always based on a player's Faction affiliation(s). Sometimes it's explicitly stated in a Role PM, other times it's assumed to be something standard. Games where the town's Win Condition was explicitly stated in the PMs of the Pro-Town players have sometimes led to non-Town players people being lynched on account of not knowing how the Game Moderator chose to word the town's Win Condition.
Note: It has been noted that the win condition sometimes used for Scum groups, "You win when you match or outnumber the Pro-Town players", is not necessarily true when a Vigilante exists on the town's side. If the Mafia's night kill is somehow stopped (by a Doctor or a Role Blocker, for example), and the Vigilante kills one of them, then a 3-3 game that appears to be won for the Scum at Twilight could become a 3-2 ProTown advantage at Daybreak. Because of this, it is important as a Game Moderator to word your Win Conditions in such a way that all possibilities are accounted for, without including possibly-spurious situations you didn't intend to trigger a win.
One possibility that is fairly fool-proof is "You win when all other Factions are dead." This puts the decision of when the players have reached Endgame back in the hands of the Game Moderator, and lets the players worry about the actual play of the game. If it reaches a foregone conclusion with no possibility of alternate outcomes, then it can be ended early.
When designing the game, the Moderator should always consider what will happen there is no obvious winner. For example if:
- A position is reached where nobody can kill anybody else (for example, if the last two players alive are a Bulletproof Townie and a Mafia Goon).
- Everyone is dead (this can happen in a game with multiple killing roles).
There is no "right" answer as to what happens in such situations - it is up to the Mod. These situations may result in a draw, or a win for a particular faction. But the Mod should anticipate such possibilities and define the player's win conditions accordingly. [For example, it is quite common for a Serial Killer to have the following win condition: "You win if everyone else is dead, even if you are not yourself alive."]