Changing is the ability to take on a different form. In the case of Eidolon demons, they are able to take the form and shape of others, and werewolves can transform fully or partially into wolves. In addition, vampires can also change into the form of rats, bats, or dust.
Demon & Shadowhunter
In 1878, Tessa Gray, the offspring of a Nephilim and Eidolon demon, was taught, and forced, by the Dark Sisters to control her ability to Change. The sisters spent hours training Tessa, rather harshly and suddenly, how to Change. They gave her potions and blackmailed her to motivate her into following their instructions.
According to Tessa, Changing was agonizingly painful in the beginning of her learning process, to the point where she was worn down and would vomit and lose consciousness. Over time, it became easier, as simple as following a series of memorized steps. She was taught to imagine her body as an amorphous and changeable form.
Given a personal effect of the person she had to Change into, she was told to "reach into" the object, imagine them as living things, draw out the spirit—the ghostly echo of its previous owner—that animated them, allow it to unravel around her and will on the Change to come.
When the Change comes, Tessa is initially in a state of darkness as her surroundings quickly fall away in her own mind. She experiences a flicker of light slowly envelope and cover every layer of her own skin until she has Changed into the mind and body of someone else. Her skin begins to tingle and sting with tiny shocks, and her whole body would shudder, as her body shifts, her bones snap and reform, her skin literally changes and reshapes. As the Change is accomplished, the pressure inside her skin builds. From an external point of view, Tessa's change is described as a ripple, "like a stone thrown into a pond," the physical change scattering and moving through the body.
Eventually, it became almost effortless, even making it possible to Change into someone a second time through the established connection, even without the person's object.
Tessa, however, is special because of her lineage. Because of her mother's angelic yet un-Marked Shadowhunter origins, Tessa's power to Change comes with abilities other ordinary Eidolons are not capable of, and it was for this very reason that Axel Mortmain wanted her born.
This Change is both mental and physical, not only in the sense that she can Change her appearance to mimic the person, but she can also touch the person's subconsciousness, allowing her to access and skim through certain thoughts and memories of the ones she Changes into. She can even experience the dead person's moment of death when she recalls it; such was the case with Adams and Emma Bayliss, feeling the pain and even being bloodied, though the Changer herself remains unharmed.
A special Change can also be performed by Tessa when she has full access to a being's soul, as was the case with the warlock John Shade, a piece of his soul seemingly kept in his watch, and the angel Ithuriel, a small portion of his spirit also trapped in Tessa's clockwork necklace. Drawing on the soul will cause Tessa to "become" the person, not in control, as if fully possessed by the soul, allowing them to speak "through her" instead of her merely speaking with their voice. This will only last for a brief amount of time, during which Tessa will not be able to undo the Change until it ends on its own.
Beings who can Change
Beings Changed into
- Annabel Blackthorn — by Sabnock's sibling
- Oskar Lindquist — by Sabnock's sibling
- Richard Gray — by an Eidolon Greater Demon
- Wolves — by werewolves
By Tessa (in order of known occurrence):
- Emma Bayliss
- Jessamine Lovelace
- Camille Belcourt
- Aloysius Starkweather
- Nate Gray
- John Shade
- Detective Abberline
- Rosemary Herondale