Lupul
Native to Ravnica. A mysterious, highly intelligent, shapeshifting creature made of maggot-like worms. Their natural form is as a massive, writhing mass of these purple worms, but they have the ability to disguise themselves as any other creature they see. Lupuls usually take the shape of their last humanoid meal and copy that creature's characteristics, powers and even memories to make them the perfect infiltrator. The Boros Angels thought they had killed all of these creatures, but a small colony survived and served under the House Dimir Lord, Szadek. Lupul is the Old Ravi word for "Lurker".
(Dimir Doppelganger, Mindleech Mass)
Read more about this topic: List Of Species In Magic: The Gathering