Du'sien is a race of shape-shifting Eidolon demons whose true form is abstract and irregular, like a greenish-gray blob with a glowing black core. Because they cannot mimic other creatures with as much detail as Eidolon, Du'sien demons are often disguised as "generic humans," or members of a crowd, rather than impersonating specific individuals.
Will Herondale briefly suspected and considered the possibility that the demonic murderer lurking in East London in 1888 was a shape-shifter, particularly a Du'sien demon, until he recognized the unlikeliness, pointing out that it would not have been able to trick anyone close to them into believing they were at all human.
After Jace and Clary were attacked by a Ravener in Clary's apartment, Du'sien demons disguised as mundane cops arrived to cover the tracks of the demons. Their disguise was imperfect, however, as one of the Du'sien demons disguised as a policewoman had a fleshless, skeletal hand. Jace had to Mark Clary to hide from them.