Breed her in a ritual

Rin took you to the underground base of the house.

In the light from the lamp, you saw a magic circle. It's the same circle you're in when you came to this world.

Rin spoke: "Let me explain something first

"Tohsaka family is a magus lineage. We practice Magecraft in order to participate in Holy Grail War, a war between magi. Whoever wins this war will get a wish from the Holy Grail.

To fight in this war, a magus like me have to summon a Servant. It's either a Heroic Spirit, the soul of a hero who achieved great deeds in life, or a Divine Spirit, the remnants of gods after the deterioration of the Age of Gods."

"Ok, but what can I do? I don't know magic."

Rin's face was blushed: "The benefit a Servant got from this is to be reborn. Thus in order to summon a Servant, the Master must be a female Magi. She will mate inside this summoning circle. If she becomes pregnant, the ritual succeeds and a Servant will be called to this world. When the war ends, that Servant will be reincarnated to the child.

And that's what I'm asking you to do. Can you help me?"

You smiled: "I don't think there is any man who can refuse such an offer, especially when it came from a beauty like you."