Unlike other homunculi who are born without eggs and wombs, Illyasviel originally was developed in her mother's womb from a fertilized egg to an embryo through the same process as ordinary people do.
As such, Illya was prepared by her family to act as a living vessel for the Grail. Illya playing with Kiritsugu for the last time before the Grail WarĮinzbern, one of three families of Magi involved in the Holy Grail War, were responsible for creation of the vessel into which the souls of the defeated Servants would go.
