The Darkness (5e Other)
From D&D Wiki
The Darkness is a secret assassin's guild that only kills evil-doers or evil creatures. To be a part of this guild you have to be of Neutral Good or Chaotic Good alignment. Duma is "Neutral Good" alignment and thus wants to do good things but isn't afraid to break the law to do so. The Darkness assassinates evil-doers such as slave-traders, sexual abusers, evil overlords, monsters terrorizing cities, etc. The Darkness exists to rid the world of very evil people and creatures.
It's possible for NPC's to not be assassin's but rather informants for The Darkness.
To find out if someone is a member of The Darkness, you can ask them using a callsign. If you ask "are you feeling lightheaded?" and they respond, "I am not, the darkness keeps me clear", then they have successfully acknowledged they are a part of The Darkness. if they respond in any other fashion, they are likely not a part of this guild. The members of The Darkness usually don't know of each other ahead of time to help keep their members secretive. If you are an informant for The Darkness it does not mean you are a member or an Assasin, simply a contact for information and shelter. A member of The Darkness does not need to have one specific class, race or background. They just need to be willing to eliminate evil-doers for the purpose of ridding the world of very bad people.
Guild Members found here.