(n.) One who foments; one who encourages or instigates; as, a fomenter of sedition.

agitator agitprop capper catalyst come-on man decoy demagogue exciter firebrand fomenter incendiary inciter inflamer instigator mischief-maker plant provocateur provoker rabble-rouser ringleader rouser seditionary seditionist shill stool pigeon stoolie troublemaker urger