Kanshou and Bakuya

Ganjiang and Moye.
The yin and yang twin swords that Archer likes to use.
They were made by the famed Chinese swordsmith Gan Jiang, during the Spring and Autumn period.
Gan Jiang was ordered by King Helü of Wu to cast him a worthy sword, but he found himself unable to make one that lived up to his own lofty standards and soon grew frustrated.
Unable to merely stand by and watch this go on, his wife, Mo Ye, wagered her very life.
It is said that to create a masterpiece that treads on the domain of gods, a human sacrifice is necessary.
So it was that Mo Ye, standing before her beloved husband, cast herself into the furnace and used her own life to blend together the iron essence of the Five Mountains and the metal efflorescence of the Six Valleys.
In the midst of his grief, Gan Jiang completed two swords: Ganjiang and Moye, twin swords embodying the concept of yin and yang.
Because they were created using the finest ingredients and a human life, not only is their ability as swords quite high, but they also have a strong mystical aspect.
Perhaps due to some design of Archer's, words that appear to be charms against evil are carved on the blades.

Fate/side material: Fate Encyclopedia