The Third Magic, also referred as Heaven’s Feel, is one of the five remaining True Magic.
The Third Magic was originally achieved by the Einzbern family, but its secret was lost one thousand years ago. In relation to the other Five Magics, it is said the Third “showed the future”, and that it would have been best if its discovery had been the last. For the next eight centuries, the Einzberns sought to recover it until finally coming up with the concept of Holy Grail War. There is no information on the original practitioners of the Third Magic.
The soul contains the memories, mind, and magic circuits of a person, and it uses the person’s body as an anchor to the world to keep from being dispersed and returning to Akasha. Once the brain and body are destroyed, it is impossible to restore a person’s dispersed soul, and even the magical ability of the Holy Grail can’t restore a destroyed body. Even if a magus manages to place their soul into a new body, it will either be an inferior copy compared to the original, or the soul itself will eventually begin to rot.
Heaven’s Feel is a magic that allows for the materialization of the soul, which stops the inevitable dispersion of the soul once it no longer has an anchor to the world, and essentially makes it transcend to a higher form of existence. It is a magic that realizes true immortality by making the soul into a high-dimensional, planar being capable of interacting with the material world as a mental body without having to return to Akasha. The practitioner will acquire an unlimited source of magical energy due to the soul becoming analogous to a perpetual motion machine.
The Great Grail uses several parts of Heaven’s Feel in an attempt to recreate it in its entirety. Using an incomplete version, it takes a raw soul, raw energy made from spiritual particles, and copies information of Heroic Spirits in order to generate Servants. Rather than being actual resurrection, it is the temporary formation of a soul that doesn’t require a real body, meaning that they are not truly brought back to life. They need a constant connection to their masters’ prana to keep from being erased by Gaia, the will of the planet. After a Servant is killed, the Holy Grail temporarily stores their souls before they are returned to Akasha.