Use fate stay night as the basic background with slight changes.
It all started with the fire eleven years ago.
The murderer rescued the surviving girl in the ruins of the church.
The story begins here.
Madara (Archer): It is precisely because that little body has endured so much pain and sorrow that it is even more lovable!
Sasuke (Saber level): Even so, I must get the Holy Grail. If I can't get it, I won't even have the chance to apologize to you.
Tobirama (human): Is this the ending? Hahaha! I, who took advantage of others by alienating others, actually died when someone alienated my brotherhood? ... But... This ending suits me just right!
Hashirama (Rider level): You can't stand up to me now!