House Dimir, the Unseen, the Tenth Guild -- a figment, a tale told to children to keep them in line. According to folklore, the vampire lord Szadek attended the signing of the Guildpact as a secret tenth signatory whose presence and existence were hidden from view. Over the millennia, ghost stories about the Dimir grew more and more complex, telling of ancient, undead necromancer-advisors, phantasmal assassins, and slick, black horrors slithering through the endless maze of sewers under the city. If you listened to Ravnica's more suspicious and paranoid denizens, you might have come to believe that House Dimir's agents were every where, all serving as Szadek's eyes and Ears. Also known as: The Unseen
Guild Leader: Szadek, an eldritch vampire
Guildhall: Duskmantle, a hidden castle inside a huge underground chamber. Only it residents and their underlings know its location.
Values: House Dimir wants utter control of Ravnica. Such absolute control requires complete invisibility so as not to arouse opposition. Therefore, Dimir works very hard to ensure that Ravnicans don't believe the guild exists.
Structure: Isolated cells. Each Dimir agent works in almost complete isolation. Often, any operative within the guild has just one contact, so that none have too wide a view of its dealings. Dimir leaders issue commands through messenger spirits or other magical means to preserve anonymity.