Eidolon is the term generally used to refer to a variety of around a dozen shape-shifting species of demons.
Eidolons are shape-shifting species of demons. An Eidolon demon's true form is white, greasy, and slimy like a slug, with its body covered in gray pockmarks. A known subspecies of the demon are the Du'sien.
Due to the nature of their ability, Eidolon demons have become the dominant means by which warlocks are conceived since the Incursion. Additionally, while in the form of a human, Eidolons are somewhat able to endure the diffused light of the sun.
In 1857, an Eidolon demon attacked the carriage of Linette Owens and her servant. It had seemingly devoured the driver and was going for the servant when Linette fought back. From a distance, Magnus Bane and Edmund Herondale witnessed the events. Edmund rushed over and almost immediately slew it. Linette mistook the demon, in the form of a man, to be an evil faerie.
In the early 1860s, the Eidolon Greater Demon Belial, in the guise of Richard Gray and under Mortmain's instructions, slept with Elizabeth Gray, who then bore the half-Shadowhunter, half-Eidolon demon Tessa. Tessa was trained to Change by Mrs. Dark, an Eidolon demon employed by Mortmain. Because of her Shadowhunter blood, Tessa is a special kind of Eidolon, as she can "become" what she transforms into, able to touch the minds of those she Changes into, living or dead. She recalls their memories and can even feel their emotions, although she described the process as a painful one.
It was also an Eidolon demon that Clary Fray witnessed Jace, Alec, and Isabelle Lightwood kill at the Pandemonium Club in 2007. It had been lurking in the club in the form of a blue-haired mundane boy, looking for victims, his blade glamoured to look like a wooden beam. Isabelle lured it into a storage room and her brothers ambushed the demon. The demon tried to bargain for his life with information about Valentine Morgenstern but was ultimately killed. Du'sien demons, one of the lesser-known Eidolon demon species, then came to the home of Clary and Jocelyn Fray disguised as police officers the night the latter was abducted.
In 2012, Prince Oban of the Unseelie Court had an Eidolon demon in a cage hanging from a tree at one of his revels. Oban presented the demon to Horace Dearborn of the Cohort, and Dearborn had him replace Oskar Lindquist to attend the war council at the Los Angeles Institute. Otherwise, the demon was in the form of Annabel Blackthorn when meeting with Dearborn, and later when it confronted Julian Blackthorn at the Imperishable Fields. Though the demon was reinforced with magic that made it harder to kill with typical Shadowhunter means, It was later destroyed by Emma Carstairs.