You were a Master participating in the seventh Holy Grail War. You had written a magic summoning circle in front of the alter in a church. After saying the summoning incantation a large beam of light arose and with a shade of mist stood a blond haired women in knightly armor.

Artoria: “I, King Arthur and Saber class Servant ask you… are you worthy to be my Master?”

The mist faded as you got a better look at your Servant, she gave off a vibe of pure nobility and kingship.