The hallowed scabbard of Excalibur, the embodiment of the utopia King Arthur seeks, originally stolen from her shortly before the Battle of Camlann. It is later used as a catalyst to summon Arturia during the Fourth and Fifth Holy Grail War.
Avalon is the lost scabbard and third Noble Phantasm used by Saber to sheathe her sword Excalibur. Created by fairies, the scabbard blessed Saber with immortality during her time alive as King Arthur. The sheathe was excavated and used as a medium by the Einzbern family to summon Saber for the Fourth Holy Grail War. Kiritsugu Emiya, Saber's Master during this time, had hid its existence from Saber and stored it within the body of his wife Irisviel throughout the Grail War's events. It was later transferred to Kiritsugu's body, where it served as a valuable asset during his heated battle with Kotomine Kirei in the Grail War's final moments. Following the Holy Grail's destruction, Kiritsugu later transferred the scabbard into Shiro's body to save his life when he was fatally injured from the Grail's destruction and was unintentionally used by the boy to summon Saber for the 5th Holy Grail War. Upon learning of Avalon's presence during their first battle with Gilgamesh, Shiro would give it back to Saber for her personal use against the King of Heroes.
When a person has Avalon in their possession, they gain rapid healing abilities that allow them to regenerate quickly from fatal injuries. The potency of this power depends on one's proximity to Saber as the scabbard draws upon Saber's mana reserves to use its abilities and its powers aren't as effective if Saber is too far away from Avalon's present owner. In Saber's possession, the scabbard allows her to manifest a barrier that isolates her from the physical plane of existence and protects her from destructive effects directed towards her from the physical plane, parallel worlds and multiple dimensions.