"I don't really know all the details, but I guess sometime before the Fourth Fuyuki Holy Grail War, Angra Mainyu, All the World's Evil, got to the grail and corrupted it. That's why instead of granting the winner's wish when it manifested, it cause destruction instead, I think."

Ritsuka's a bit fuzzier on the technical grail war stuff, since she's never been particularly interested in the ritual. But she at least remembers this bit of information coming up.