Avalon

The hallow scabbard of Excalibur, originally stolen from King Arthur shortly before the Battle of Camlann. Mere possession of this Noble Phantasm bequeaths limited immortality through regenerative stimulation, as well as preventing physical deterioration due to aging. When initiated as a Noble Phantasm, Avalon dissipates into countless particles and immerses its user into a tranquil domain of the fairies, virtually a "portable fortress". While bounded in this manner, an individual is shielded from all destructive interference in the physical realm, from parallel worlds, and dimensional planes. Avalon transcends even the most advanced sorceries, and is comparable only to "true" magic. The name alludes to a legendary isle in English folklore, best known as the final resting place of King Arthur.

Addtional source: Fate/Stay Night game translation v1.0a by Mirror Moon