Avalon

The Ever-distant Utopia.
A sheathe crowned with the name of the land of fairies that appears in the legends of King Arthur, said to be wreathed in everlasting spring.
It is a paradise reminiscent of the island in Greek mythology where the gods grew their golden apples of immortality.
Not only does the holy sword’s sheathe heal all of its owner’s wounds and stagnate their aging, but when its true name is called it even decomposes into hundreds of parts and protects them from all interference.
It was excavated from Cornwall by the Einzberns, and entrusted to Kiritsugu as a catalyst for summoning King Arthur.
It is a Noble Phantasm at the level of magic, that completely shuts out all physical interference, transliners from parallel worlds, and communication from other dimensions (up to the sixth).
If Saber secludes herself with this, none will be able to reach her.

Fate/side material: Fate Encyclopedia
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The hallowed scabbard of Excalibur. Rank EX Noble Phantasm. The name “Avalon” comes from the land of the fairies in the legend of King Arthur. The Everdistant Utopia (Avalon) heals its bearer and prevents aging when infused with magical energy. When employed as a Noble Phantasm, it shields its wielder in the land of the fairies, bestowing absolute defense which nothing may interfere with. This feat is in the realm of true magic. The 1500 year-old scabbard was dug up in Cornwall at the behest of the Einzbern family. Emiya Kiritsugu uses it to summon Saber.

Translator’s Notes
  1. ^ The Everdistant Utopia.
Fate/Zero Animation Guide I: Glossary of the holy grail war I
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Excalibur’s sheath. It prevents aging and provides unlimited healing and restoration. Rank EX barrier-type noble phantasm. Irisviel uses the power of the sheath to stave off the collapse of the “shell” that is her herself. The sheath is eventually passed to Kiritsugu to mitigate the damages that Innate Time Control causes to the inside of his body, as well as instantly restores Kiritsugu after he takes a blow from Kirei that would otherwise spell instant death. Kiritsugu later embeds the sheath in Shirou, saving his life. He adopts Shirou after this event. The connection of the sheath also contributes to Shirou being able to summon Saber in the Fifth Holy Grail War.

Translator’s Notes
  1. ^ The Everdistant Utopia.
Fate/Zero Animation Guide II: Glossary of the holy grail war II