do we want to go ghost or something you can touch like bigfoot.
What about: Beneath every statue of southern heroes from the civil war a slave is buried.
Antifa probably already believes this, tbh. Do you want them to hunt down statues even more zealously?
It is a downside to spreading that myth that is true. And we would need a good followup plan if we are to spread it and turn it into our benefit.
another option is to go for an event, like if somebody does this and that at this time and location, then (urban legend: event) will happen.
The legends speak of rituals in which cultist would scream their chantings (allahu ackbar) on key, sacred places (in front of police stations) to plant the seed of chaos in the world.
While some of them succeed at making the mince hide (cause crowd panic) most of them only do so to find their fate cut short by blows from the agents of order, mysterious protectors that pray for the order (they get shot by cops/arrested)