The Mermaid

Kiara/EMIYA. There are many timelines. In one, Shirou Emiya accidentally summoned Kiara Sessyoin to the Fifth Holy Grail War. When she induced him to do a mana transfer, he performed so poorly even Kiara was turned off. These events are documented... elsewhere, by other talented people. (If you wish, please seek out The Saga of Shirou's Summons as well as A Long Night, Short-Lived, on another fanfiction site.)

This is the story of what happened shortly after that ill-fated encounter, when a much older version of Shirou Emiya returns to square the reckoning.