The Decider
If any anti-town player dies at any point, you cause all members of their faction to die too; the rule of "if an anti-town player dies, so does their faction" will be publicly announced at the start of the game.
At the start of day 2, each anti-town faction with more than one member gains a 1-shot factional day self-treestump ability (the player who uses it becomes permanently deathproof, voteless, alignment-confirmed, immune to prodding/activity requirements, and ceases to block other player's win conditions); and you and each townie gain an (unlimited shot, but only one use per day) All-In Desperado day action (basically a dayvig, but if it hits a townie it backfires and kills the person using it
and that person becomes a Survivor automatically loses
; so it's a pretty big risk!). At the start of night 2, each anti-town faction gains a factional kill (or an additional factional kill if they already had one, allowing them to kill twice in a night), which persists throughout the rest of the game.
You win when all anti-town players are dead.
Based on a mechanic by zoraster, modified for non-Mountainous setups (which a Grand Idea probably will be).