One who stirs up trouble between others.
Story of the Signifyin' Monkey who tells the lion (King of the jungle, that in a nearby well, there is a potent adversary. The lion looks down into the well, sees his reflection and growls. When he takes the echo of his own roar to be a challenge he jumps into the well to attack the supposed adversary.
This defuses his power at no cost to the monkey.