The Assassin Organization is a group of mercenaries based in Gusteko, led by an individual known only as Mama.
The organization operates by receiving requests from clients, such as assassinations, and tasks members to fulfill said jobs. So far, members of the organization have committed murder, assassinations, attempted to steal an insignia, and caused a Demon Beast incident within Roswaal's territory for an unknown purpose.
Mama's identity is apparently a secret, as Meili didn't know what the former looked like. Also, Mama doesn't tolerate failure, as Meili stated she would be disposed of if she returned or was found by the organization.