User: Saber
Excalibur’s appearance after being corrupted by evil. Excalibur functions by converting its user’s magical energy into offensive power. Because of this nature as a “sword that amplifies”, when Saber was blackened, Excalibur blackened with her.
In Arthurian legend, King Arthur received Excalibur from Vivian, a fairy of the lake. Just as the fairy Vivian who embodied goodness had her polar opposite in the evil Morgan, so too does Excalibur have the possibility of embodying both good and evil. However, whether Excalibur is filled with good or stained by evil, the fact that it is the strongest holy sword does not change.

Fate/complete material II; Weapons: Black Excalibur, pg. 121