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