An Arsonist is a scum role which works similar to either a Serial Killer or a Poisoner. In the former case, it is functionally equivalent to an SK, with the exception that it is almost always paired with a Firefighter that protects from Arsonist nightkills. The latter is as follows:
Typically, an Arsonist targets a player to "Douse" their home with gasoline, acting as their primary night action. However, at certain points in the game, the player may decide, in lieu of dousing, to instead "Ignite," killing all Doused players at once. This has the drawback of sacrificing a nightkill compared to a normal Serial Killer, but has the benefit in that the sudden girth of dead players can not only frighten the town, but cause many of their assumptions regarding the setup to be reconsidered.