Necrodermis

Necrodermis is the xenos material created millions of years ago by the Necrontyr species that is often described as "living metal." It was originally used by the Necrontyr to construct their massive sub-light starships that explored and settled the Milky Way Galaxy millions of years ago. It was later adapted to create the robotic bodies inhabited by the C'tan and the Necrontyr after they agreed to have their consciousnesses transferred from their short-lived organic forms, This process transformed the Necrontyr into the undying Necrons.

Necrodermis is a material of unknown origin and structure that possesses the extraordinary ability to regenerate almost all damage instantaneously, "flowing" back together as if it were a liquid while closing bullet holes, mending gashes and tears, or even reattaching severed pieces with little delay. The material is also adaptive in some unknown fashion and can learn to repair itself given enough time from nearly any form of damage, even a blast powerful enough to reduce it to its constituent molecules or atoms. In addition to the bodies of the C'tan and the Necrons themselves, all Necron vehicles and starships are made from Necrodermis, including Monoliths and Necron Pylons.

C'tan
The Necron Star Gods, the C'tan, are entities of pure energy that can only interact with the physical universe through the possession of robotic bodies crafted for them by the ancient Necrontyr from Necrodermis. The shape of their Necrodermis body can be modified at will by its possessing C'tan, and so each Star God looks very different from its fellows. C'tan can also shape powerful slashing or cutting blades from their bodies at will or shapeshift in such a way that they can appear to be a member of almost any species. If a C'tan's Necrodermis body is breached, the C'tan essence is released with a powerful explosion of energy that can damae nearby individuals or structures. But the C'tan is not slain, only forced to wait until its Necron thralls can forge a new necrodermis body for it to possess and return to the material realm.

C'tan Phase Weapons
The C'tan Phase Weapons used by the Imperium's Callidus Assassins are also made of Necrodermis. These weapons were adapted from the remains of Necron technology discovered to lie buried dormant under Mars by the Adeptus Mechanicus many millennia ago. Their blades can move out of phase with the space-time continuum much like the Necrons themselves do and so cannot be blocked by any defence save for another weapon crafted from Necrodermis.

Asirnoth
It has been surmised by Imperial savants after the Imperium of Man made first contact with the Necrons that Asirnoth, the strange Silver Wyrm that the Primarch of the Iron Hands Legion, Ferrus Manus, battled on the volcanic world of Medusa over 10,000 Terran years ago was a Necron construct created from Necrodermis. When Manus' hands were coated with a silvery metal that melted from the Silver Wyrm's body, that material was absorbed into his skin and became a part of his body rather than injuring him. This would accord with the way Necrodermis has been observed to act by Imperial scholars.