A black firework exploding
Black Fireworks are used to alert other Villager Scouts to avoid level 10 threats, and alert Villager Knights to assist them.
According to Joe, You have to use two black fireworks and survive to achieve Captain Status of the Villager Scout system, meaning you have to survive 2 level 10 threats.
A level 10 threat, as a villager knight was defeated here, alerting scouts to avoid it.