Flesh Manipulation

Flesh Manipulation is a supernatural ability possessed by demons that allow them to alter and control their own flesh and body.

Overview
Flesh Manipulation, which may sometimes be referred to as Shapeshifting, is an ability which is shared among all demons, and enables them to influence and change certain physical traits. For lesser demons, any and all changes to their biology is mostly involuntary, and presumably occurs during the demonification process where they undergo monstrous mutations, growing horns, pointed nails, fangs and jaws, prominent miscolored veins around the forehead, and usually some kind of marking which resembles a birthmark or tattoo somewhere on their body.

In some cases, lesser demons do have some form of control over their own body, like when the Temple Demon grew arms where his neck should be, and the Hand Demon growing numerous arms and being able to control all of them. Some lesser demons also have special body parts, such as the Tongue Demon who has a tongue sharp enough to cut through a vase. Some may also involuntary gain animal-like characteristics, such as the Serpent Demon and the Elder Brother Spider Demon.

It is implied that as a demon grows stronger, their control over their own flesh increases as well. Members of the Spider Family, which are empowered by the blood of Rui, a member of the Twelve Kizuki, can form basic objects using their own body, such as silk and string that act exactly as they should.

Members of the Twelve Kizuki possess even greater flesh manipulating prowess. Rui can create thin threads out of his own cells that are sharper than blades, while Hairo can turn his body into a "Hammerspace" to store equipment. More impressively, Enmu can fuse his entire body with an object as large as a train, turning the entire locomotive into his flesh, blood, and bones, and forming tendrils and appendages to freely attack all those inside the train. Enmu is also able to freely shape parts of his train body to create constructs to aid him.

Twelve Kizuki members such as Daki and Hantengu's clones have been shown to be capable of creating all kinds of clothing and accessories that are indistinguishable from their normal counterparts, and function and behave exactly as how they normally would, with the sole exception of burning up in the sunlight or upon their death.

Moreover, they can also create weapons out of their own flesh. Hantengu's clones have created a Khakkhara, a maple leaf-shaped Uchiwa, and a Jūmonji Yari. They can be used in combat exactly as how their normal counterparts can possibly due to their own body's unnaturally high durability which can also result in their weapons being more high quality or durable than those used by the Demon Slayers. This flesh-weapon combination have the added benefit of being repaired if damaged like how demons would regenerate any other part of their body, and can also be created at will if it is destroyed or lost.

Some demons' flesh manipulation is of such high fidelity that they can use their flesh and blood to mimic elements such as ice, water, lightning, and wood. They function and behave exactly as they naturally should, with the added benefit that they can be regenerated and created at will.

The flesh manipulation of Muzan Kibutsuji, the Demon King, is perhaps the most potent, being capable of mending entire organs to the point of functioning with 7 hearts and 5 brains inside his body. He can also form orifices such as mouths on his limbs, as well as altering the shape of them at will. He can even create identical clones of himself that he can remotely command. His shapeshifting has also been called extremely precise, as even the Lower Ranks were unable to recognize him.