Third Magic

The Third Magic, also referred as Heaven's Feel, is one of the five remaining true magics in modern Nasuverse.

History
The Third Magic was originally achieved by the Einzbern family, but its secret was lost one thousand years ago. For the next eight centuries, the Einzberns sought to recover it until finally coming up with the concept of Holy Grail War.

Effects
The Third Magic is described as a materialization of a (human) soul. Generally, a soul is kept anchored to the material world only for as long as its respective body and brain stay alive. The moment their life functions cease, the soul disperses and returns to Akasha to be recycled for the next incarnation. The process is untenable except by using the Third Magic.

While not directly creating a new body for the soul of a deceased person, Heaven's Feel anchors it to the material world for an indefinite time span, stopping its dispersion and allowing the caster to create a new body for the subject. The key difference between a soul attached to the body in the "natural" way and a soul subjected to the Third Magic is that the latter doesn't need a physical form to continue existing in the material plane, i.e. making it a higher form of existence.

Consequentially, the Third Magic is the only way to achieve perfect resurrection and immortality without loss of personal abilities and soul integrity.

As a Magic, by definition, Heaven's Feel is a process that cannot be scientifically explained. However, explaining what it isn't can bring a better understanding.

In the case of the Holy Grail War, the Great Grail utilizes several parts of the original Heaven's Feel magic in attempt to recreate it in its entirety. Specifically, the Great Grail system uses a raw soul to copy informations of a Heroic Spirit.

However, it isn't an actual resurrection, and it isn't the real Third Magic. To begin with, the summoning of the Servants doesn influence their lifespan: they are still supposed to be dead in the present world. For that reason, Gaia considers Servants to be an interference, a contradiction which it must crush. Hence, the Master needs to continuously provide prana for the Servant to remain in the present world. Moreover, Servants don't have an actual body, so as a soul, they should normally disperse and return to the Akasha. When the Servant is killed, it should again disperse and return to the Akasha, but the Holy Grail is created to store them temporarily before that happens.

Additionally, the mud from the corrupted Grail grants the Servant a body, but the Servant is still considered as "not alive", an interference, by the World: the Servant still needs prana to remain in the present world.

With the above in mind, the real Third Magic is, simply put, the miracle that allows a materialization of the soul, in the sense that it can stop the inevitable dispersion of the soul. The soul can then be given a body (by any suitable mean); afterward, by nature, as long as it has a brain to be fully anchored to the material world and a body, the soul can reshape the body according to its memory. In that regard, the materialization of the soul makes it a higher form of existence.

Practitioners
No information on the original Einzbern practitioners of the Third Magic is available. If it can be considered to have any user in modern times, it would be Lizleihi Justizia von Einzbern, who is the core of the Great Grail. Illyasviel von Einzbern also makes use of it in one of the routes of Fate/stay night, although she couldn't use it properly.