Avalon

Avalon: The Everdistant Utopia is the hallowed scabbard of Excalibur, the embodiment of the utopia King Arthur seeks, originally stolen from her shortly before the Battle of Camlann due to the machinations of Morgan le Fay. It is a luxurious piece of equipment made of gold and decorated with blue enamel that seems more like a treasure to show dignity and nobility like a crown or staff than a weapon. No one knows what kind of a divine mystery it is made from, but it was created by fairies along with Excalibur, an inscription written in Fairy Letters engraved on its center to show that it is not the work of man. It bequeaths limited immortality through constant regeneration, as well as preventing physical deterioration caused by aging. The utopia Avalon was thought to be derived from the island that produced the "Apple of Immortality" in Greek mythology.

The Noble Phantasm is not like one brought forth from the Throne of Heroes with the Heroic Spirit or actually possessed by Saber due to her unique status. It is the genuine artifact that was lost to her, excavated in Cornwall at the request of the Einzberns shortly before the Fourth Holy Grail War. It is still in pristine condition after one thousand five hundred years, though it no longer functions without a connection to its original owner. Used as a catalyst to summon Altria into as a Saber-class Servant, its existence is kept hidden from her. It is later used to save the life of Shirou Emiya at the end of the Fourth Holy Grail War, and it is once again used as a catalyst to summon her during the Fifth Holy Grail War.

The holder of the scabbard is granted potent healing, allowing for critical and fatal wound to be rapidly repaired to restore the wielder's health. Minor injuries are restored easily, and even large missing portions of the body and destroyed vital organs like the heart can quickly be restored at the critical moment before death. Targeting anything other than the holder's head in order to destroy their brain is futile, requiring for a decisive strike to be landed in order to cause any true damage. It does have its limits in repairing the body, as counteracting the curse from Shirou being stabbed by Gae Bolg takes a great deal of time and dealing with the massive backlash from Ea clashing with Excalibur takes a number of minutes to restore Saber's health.

Its function as a Noble Phantasm is an "absolute defense" that completely shields its user in the domain of fairies, Avalon, the unreachable utopia that Arthur dreamed of and was said to have gone to after her death. It is the greatest protection in the world that goes beyond defending or reflecting, completely isolating its user in a world completely separate from the regular world. The scabbard dissipates into countless tiny particles in the air and engulfs the user to become a "portable fortress" that shuts out all interference. It is the Bounded Field of the tranquil domain of fairies that keeps out all filth from the outside world, and allows nothing to harm the tranquil King that stands in the land of Avalon. The individual is shielded from all destructive interference in the physical realm, transliners from parallel worlds, and multidimensional communication as far as the sixth dimension. It is on the level of true magic, an actual true magic in itself, that transcends all magecraft, and not even the Five Magics can overcome the barrier.

It is said in Arthurian legend that after Arthur received Excalibur Merlin approached him with a "magic sheath" that would not allow for a drop of his own blood to be spilled so long as he possessed it. It can be called even more valuable than Excalibur due to its loss being connected to his death. When asked "Your highness. Which do you like better? The sword or the sheath?", Arthur is said to have answered that the sword is better without hesitation. Merlin scolded him instead with "Please make no mistake here. The sword slashes the enemy, but the sheath protects you. As long as you have the sheath on you, you will spill no blood and take no wounds. You should truly value the sheath, not the sword."

Master's Noble Phantasm
While normally useless as a magical artifact, Avalon can also be used as a Master's Noble Phantasm after Saber is summoned. It can be disassembled and sealed inside Saber's Master or someone who will constantly be close to her, such as Irisviel von Einzbern, as a Conceptual Weapon to aid them with its regenerative abilities and allow them to supply more prana to Saber. The contract between Saber and a Master isn't a requirement to use Avalon's abilities, but having one will provide the user the true benefit of limited immortality by allowing it to obtain prana from Saber and fully utilize its effects according to the contract. This allows Kiritsugu to use Innate Time Control, which causes enough damage to shatter bones and tear tendons with brief use, at double his normal level for extended periods of time. However, it only nullifies the damage he receives; he is still burdened by the excessive pain of the injuries.

Avalon's effects are less potent for those without the contract, allowing its regenerative abilities to be used only while Saber is close enough to provide prana to it. It will not function at all if she is unable to provide it with prana by being near its bearer. Its functionality increases as she gets closer and becomes able to provide it with more prana, and it can be increased further through direct contact with the wielder. It is enough to keep Irisviel functioning as person, while she would normally start to break down and lose functions as a human due to her status as the vessel of the Grail. She needs to be within range of Saber to function normally, while her suffering increases as Saber moves farther away.

Avalon can be removed from its user at any time to be passed on to another. Irisviel is able to manifest it by placing her hand on her chest, concentrating prana within her on to her fingertips, and allowing the form of the scabbard to appear within a silhouette of light before finally manifesting. Kiritsugu is able to do the same to implant it into the dying Shirou, and it remains within him for many years. It remains merged with him in Unlimited Blade Works and Heaven's Feel, but it is discovered during the Fate route after and Saber manage to project it to reflect the strike of Merodach. Shirou and Saber must employ a different method to retrieve it from within him, having Saber to gather its form while Shirou shapes it with Projection in order to return it to her from across time.

Though Saber is already gone when Kiritsugu implants it within Shirou, the residual energy residing within it is enough to resuscitate him. It becomes inert afterward, but it draws him closer to swords and actually changes his Elemental Affinity and Origin to "Sword" as well. Once the Fifth Holy Grail War enters the preparation stage, magical energy starts flowing within Avalon due to it being confirmed that Shirou will summon Saber. After it is used as the catalyst to summon Saber a second time, the connection is fully re-established and it heals him from a number of fatal wounds during the War. Even after separating it from him, it has merged with him to the point where he is able to create a perfect projection of it without any degradation that bypasses all of the steps due to having a complete record of it. After the connection to Saber is gone, it can be said his image of Avalon also disappears.