@scripted_2000 It's explained in the Ishtar chapter. Basically Shirou and Rin had some beef, and they had a showdown. Of course Shirou was winning, but he decided to purposefully lose and die by the hands of Rin to fulfill her "revenge". Now after dying, his magecraft, projection gets ascended to the throne of heroes, but because it only has Shirou as it's sole user, the sould of Shirou was embedded on the projection craft essence. This isn't possible in the nasuverse, but I made it up. I hope this explains it. If you want more information read the Ishtar chapter, since it explains it more.