Eidolon is the term generally used to refer to a variety of around a dozen shape-shifting species of demons.
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 slayed it. Linette mistook the demon, in the form of a man, to be an evil faerie.
An Eidolon demon, in the guise of Richard Gray, and under Mortmain's instructions, slept with the man's wife, Elizabeth, who then bore the half-Nephilim, 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. 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 was able to lure it into a storage room and was ambushed by her brothers, and followed by Clary. 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.
- Since the Incursion, Eidolon demons have become the dominant means by which warlocks are conceived.
- Eidolon are somewhat able to endure the diffused light of the sun when in the form of a human.