A silver medallion bearing the emblem of a moth resting on the blade of a dagger. Used as a signifier for Thieves Guild membership.

“It’s a token of the local thieves guild, and those are the initials of its previous owners. It might grant you access to new parts of Undercity.” - Zalic, Outbreak in Undercity (RPA-ADV3)

You pick up the medallion after an encounter with Mowla, Bastja follower of the Starlit Door, at Azema's Bones, and are able to gain entry into the Thieves Guild with it;

“Steal a medallion from a thief. That’s the only way to become a member of the guild. Go on in.“ - Thieves Guild guard, Outbreak in Undercity (RPA-ADV3)