Ravnica style guide: Dimir skeleton troops and Dimir House Guard
Combine Blue's mastery of metamagic and Black's ready access to undead troops, and you get the Dimir skeleton: a enlightened regenerator that aids its master find a more appropriate spell when necessary. Note how the cool hovering skull details suggests a more mystically empowered undead minion.
Ravnica style guide: Thieving spirit sketch and Dimir Cutpurse
The Dimir walk among us, unseen but detectable through what they manipulate and steal. It's tough to see in this screen-width (click here for Kev Walker's full art), but the Dimir Cutpurse has its ghostly, rags-dressed arm right in an unsuspecting Ravnican's pocket. Creepy.
Exoskeletons. Ribcages. Assassin's blades made from sharpened bone. There's a lot of emphasis on exposed skeletal structure -- a dramatic twist on the secrecy of the guild. It suggests that, if you were ever to actually catch a Glimpse the Unthinkable of Dimir Infiltrator or Roofstalker Wight, you'd be out of your mind with horror. Right before you died.