Blitzyviel

Avalon is the Island beyond the Lake.

It is the eternal resting spot of the king. Coincidentally, that was the name of Excalibur's sheath, given such a name by the Lady of the Lake. She proclaimed that the sword may be the sigil of justice, but it is the sheath that bears the right as an honorable king.

In Stay Fate (this game), Avalon is represented in two forms. Avalon - The sheath and regenerative abilities, and True Avalon, which displaces Saber, rendering her invulnerible etc as she is protected by the isle.