Soul

The is a record that acts as the blueprint of the body.Fate/stay night - Heaven's Feel route - Day 7: dialogue lost loop - Interlude 7-2: Madness
==Nature==
Composed of spirit particles or Spiritrons, the soul is the second element, one of the three essential Elements that compose a human,Lord El-Melloi II Case Files Volume 6: case. Atlas Contract (Upper) - Chapter 2 Part 4 alongside and .Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 1 Part 4 It is something independent from the body, not belonging to the same material world. It is an existence of a higher order in concept of the astral plane, acting as a record of the body containing the memories, mind, and Magic Circuits. The body itself can be called a terminal for the soul to move around, and the soul itself can be considered an invincible command structure.Fate/stay night - Heaven's Feel route - True End: Return to the Spring 
While the body itself possesses a record of its composition in its genes that allows it to heal itself, that blueprint is not applicable when a person has lost their original body. The soul itself contains a record, the "genes of the soul", that allows both the body and its genes to be restored even if they are lost, should the person take another vessel. 
Souls are the only existences that are eternal in the material world, but they are unable to remain in the world by themselves without a container like a body. Once it is fixated in a body, the soul is destined with "eventual death."Fate/stay night - Heaven's Feel route - Day 06: beautiful (I) - Scene 09: Winter castle - Ilya (II) In that state, time affects even such ethereal bodies, so the soul can start to rot, corrupting even the body's blueprint. The person inhabiting a new body can only take their own form, overwriting the body no matter its quality, and that form will degrade more quickly as the rot progresses.
In the past, in the medical community, it was thought that the body equaled life, and the body was an output device needed to power the soul. Without a living body, a soul could not be generated. However in the World of Magecraft the opposite was thought to be true: the soul is a "will" that lies in another dimension, and that will occupies a body. It is a general rule that the human body can only hold a single soul.Fate/stay night - Heaven's Feel route - Day 13: LAST TALK - Gift
It is impossible for a Magus to give shape to the soul, requiring a powerful artifact like the Holy Grail to assist them in being able to recreate a soul and give it a body.Fate/stay night - Unlimited Blade Works route - Day 3: long day, long night - First Command Spell
When the body dies, the soul disperses and gradually fades away back to the Root to be recycled. This also applies to Servants, and Magi need to feed them with Magical Energy to maintain them - something that shouldn't be doable normally on one's own, unless backed up by the Greater Grail. This applies again when the Servant is killed, but the Holy Grail is here to store them temporarily to power the Greater Grail.
Vampirization - that is, making the flesh undead - is a way to prolong the soul after the death of the body; yet, the soul still decays. Vampires need to drink the blood of humans to maintain their body, and thus, their soul.Tsukihime - Near Side: Arcueid's route - 3/ The Black Beast ITsukihime - Near Side: Arcueid's route - 6/ Eyes of Death Perception I
The core that binds the soul is the head - the brain. It is said that there are three parts required for a lifeform: the soul, the brain, and the body. The brain keeps the soul anchored to the body. The soul can reshape itself with the brain and the body. Usually, it is said that destroying either two out of these three is required to completely kill someone. In the case of a vampire, it seems the vampirization, which transforms the soul, can bypass the necessity of the brain to hold the soul: after a while, vampires can regenerate their body, and thus, their brain, to maintain the soul.
Souls are noted to have color, at least in the case of Gilgamesh and his soul that glows golden.
===Magecraft and conversion===
As their privilege of being Spiritual Beings, Wraiths and Servants, and beings like Unicorns, have the ability to convert souls into Magical Energy to act as nourishment.Fate/stay night - Prologue: Day 3Fate/Apocrypha Volume 2: Round Dance of Black / Festival of Red - Chapter 3 While Magi believe them to exist and they act as a necessary element for performing Magecraft, they have been considered hard to handle since ancient times. They are simply viewed as "things to be investigated" and "things to be moved into containers", only allowing them to be moved between vessels, rather than having any applicable use in such a state. They cannot be converted and transformed with Magecraft, and as they cannot act as nourishment to humans or be converted to anything else, the ability to harvest them would prove meaningless to anything other than a spiritual being.
It is possible for some Magi to use reincarnation, creating an offspring with their soul after their death. The process is difficult, and if they are unable to inherit all of their previous self's power, they can only be called a copy rather than reincarnation. This copy, degraded by its very nature, will be inferior to the Magus' previous self.
It is said by Magi that only one Magus has ever truly understood the soul. Other people include:
*Darius Ainsworth, invented his own style of Flash Air over a thousand years ago, he created it in order to record and store dozens of years of his life, the concept of "Darius", within Pandora's Box, as it contains a place that is not bound by the same rules of the World and so the record of his soul won't suffer decay inside it. Because of this, every version after him is but a copy of the back-up stored in the Pithos, and every time he dies a record of him in his prime is used to make a near-perfect replica. This has the side-effect of making Darius unable to die unless the Pithos is opened, making him immortal. Every time he dies he retrieves a back-up from the copy stored in the Pithos that is a perfect copy of him in his prime -- it is not a flawless process however, as though his soul escapes damage, his memories diminish with each resurrection.Fate/kaleid liner PRISMA☆ILLYA 3rei!! - Chapter 65.2
*Darnic Prestone Yggdmillennia, devoting his attention into studying the soul due to having interacted with a Servant, has found a way to turn the souls of other humans into nourishment for himself. Compared to Spirits that can do so without issue, it is a dangerous ritual for a human that can lead to death if even slightly off, and the result of multiple rituals will end in his very identity being overwritten.
*Michael Roa Valdamjong (Remake), is probably the only one who ever succeeded in turning the soul, incalculable and something that instantly disperses upon leaving the body, and processed as something that can be "transmitted";Tsukihime - Near Side: Arcueid's route - 10/ Vermilion Crimson Moon IITsukihime - Near Side: Ciel's route - 7/ Bow in the Sky IITsukihime -A piece of blue glass moon- - Moon Princess route - Day 10: "Vermillion, Crimson Moon I" it is how he acquired his nickname of . However, revival of the soul is difficult, and what really transmigrates is his tenacity more than the actual Roa.Tsukihime - Near Side: Arcueid's route - 12/ The World of the Moon As he can't actually transfer the whole 100% of his abilities, he reproduces them, and always ends up inferior to the previous incarnation.Tsukihime - Near Side: Ciel's route - 9/ Cast-off Shell
===True Magic===
The resurrection and restoration of souls are divine Mysteries that can only be performed with Magic.Fate/stay night - Heaven's Feel route - Day 10: darkwood,sleepingbeauty - Artificial Phantasm 
The Dress of Heaven can materialize a soul for a few seconds, allowing a human soul to be put into another container like a Puppet.
The Third Magic allows for the materialization of the soul, bringing about an immortal existence of a higher dimension able to take a spiritual form while influencing the physical world. Part of it is used to bring forth Servants, but since its true purpose is not to duplicate a soul perceived in the past, it is an intentionally imperfect method and materialized souls given spiritual form are not considered alive as part of the current world or timeline.Fate/stay night - Heaven's Feel route - Day 15: limited. - Truth - Antihero Angra Manyu
===Purity and weight===
The souls of humans are pure and sturdy as infants, and they are dyed with their experiences in life, each year of accumulated experience adding to it. Beings created in an already-matured state like most Homunculi have empty souls bereft of any such experience, keeping the same initial purity. Given enough time, even the soul itself can begin to rot along with the body. In the case of possession, the experience of the souls of the two will mix, becoming jammed and clashing due to a lack of compatibility. Only those like homunculi that lack such experience can do such without rejection. 
While Heroic Spirits can take in the souls of others as sustenance with impunity, humans performing the same will begin to have the precision ratio of their body and soul start to reduce, eventually leaving them as "someone else" should they continue the process enough times. While that person will share the same memories and have every detail of the person perfectly recorded, that new person will still not be "them."
Heroic Spirits are considered souls of the greatest purity among humans, said to be vast and overwhelming compared to living humans, that can fill the Greater Holy Grail with only seven. Gilgamesh in particular is noted to be worth hundreds of thousands of human souls, and he alone is enough to fill the rest of a Lesser Holy Grail when only the souls of four Servants have been consumed compared to the required seven.Fate/stay night - Heaven's Feel route - Day 12: overdose - Nightmare, awakeningFate/stay night - Heaven's Feel route - Day 13: LAST TALK - Misery 
Compared to the souls of humans, those of Heroic Spirits are completely different in all aspects, greater in purity, density, strength, and solidity. For one to possess a human for even one second is considered a miracle, and the idea of taking in souls of such density is impossible for a normal person who would be unable to endure them. With the specially created vessels of the Einzberns, they can take in the souls of seven while slowly losing human functions. With the backing of a Command Spell and the lack of his experience in his soul, Sieg is able to take in Siegfried's soul for three minutes at a time, and while Darnic Prestone Yggdmillennia cannot actually devour the soul of Vlad III, he takes it in and engraves his very existence on Lancer's soul due to a number of circumstances.
Some humans, such as Salia, possess what are described as "immortal souls since birth," souls "unpolluted, undisturbed by all evil."Fate/Grand Order Divine Realm of the Round Table: Camelot Wandering;Agateram - Blu-ray Q&A Said to be "people born of eternal, immovable purity," these individuals are described as possessing angelic souls, reflecting that angels can perform no evil deeds.Fate/Grand Order - Camelot: The Sacred Round Table Realm - Section 4: The Wailing Wall This phenomenon is different from the Origin possessed by each individual. The concept of "evil" refers to actions like “unconsciously despising another person,” “damaging your surroundings for your own sake,” and “aimlessly hurting life.”
===Other===
Other special circumstances involving the soul include:
*The Hundred-Faced Hassan was born with a compartmentalized soul despite having a singular physical body, resulting in them manifesting as multiple personalities in life much like multiple personality disorder. As each is a different soul with a unique personality, they are able to manifest in different shapes once they are no longer bound by his living body as a Heroic Spirit.Fate/Zero Volume 2: The Mad Feast of Kings - Act 8 Part 6 -102:54:10
==References==
it:Anima
vi:Linh Hồn
Category:Metaphysics
Category:Magecraft terminology
Category:Third Magic