Charlatan is an organization within Jun Mochizuki'sThe Case Study of Vanitas. While the goals of Charlatan are as of yet unclear, they are responsible for the creation of Curse-Bearers; having been actively involved in incidents involving Catherine, Louis de Sade, Amelia Ruth, Thomas Berneux, and multiple Curse-Bearers at Marquis Machina's bal masque. It seems as though Charlatan share the same ideals as Vanitas of the Blue Moon, actively distributing Malnomen through the alteration of a Vampire's true name - of similar function to the Curse of the Blue Moon - however whether or not they hold any allegiance to the Vampire of the Blue Moon remain as-of-yet unknown. Additionally, when Naenia is in pursuit of a Vampire, they find themselves lost in a thick fog, where they are confronted by the Parade of Charlatan, which is used to instill fear upon Naenia's victims and make them more vulnerable - however it is unknown whether the Parade of Charlatan is an illusion created by Naenia or simply composed of more members of Charlatan.
Despite her integral role within Charlatan - Naenia does not appear to be its leader. Charlatan One reminded her of the time they'd been allotted, which they had succeeded, during the bal masque; suggesting that there is someone else with a position of power over her who is giving the orders.
Given the way Charlatan One warned of Veronica de Sade and Machina catching on to them, it seems as though Charlatan has need to fear the two; however the reason for why this would be is left ambiguous.