>>527430552
This isn't really explained in the movies but the Shadow is the corrupted grail fused with Sakura's imaginary numbers which is why its kind of an infinite space that can eat everything. Usually the corruption is just the mud that spawns in other routes and Zero.

> Could Gil have killed Sakura if he wasn’t caught off guard or was she invincible at that point?
Maybe if he pulled Ea but he would never do that anyway