Fate/stay night - 2x19 - Idealism's End (The Answer)- Followmy.tv

Shirō, Saber, and Lancer arrive at Illysaviel's villa, where they are met by Archer, who exposits on the true nature of his summoning: Rin had not in fact summoned him by chance, and the catalyst for his summoning was actually the pendent he carries, identical to the one Rin used to save Shirō. While Lancer runs off to retrieve Rin, Saber demands to know why Archer is so intent on killing Shirō. Archer responds that, like Shirō, he aspired to become a hero of justice and actually fulfilled that ideal, but became disillusioned by it and became a Servant so he could rectify it. He then forces Saber to reveal her back-story to Shirō: her true identity is that of King Arthur, who hoped to use the Holy Grail to undo her decision to become the king of Britain, as she believes it was her rule that eventually resulted in the country's destruction.