We've been keeping mum on the spoilers at
@daniel_gudman's (hey, tagging actually does something here, isn't that nice?) request, yes.
But yeah. Actually, when you think about that, that is
terrifying. Sachi is casting on instinct, in a soul container that isn't her own, with no practice and only desperate intent, inside what is basically an absurdly powerful Bounded Field that defies all damage - which by this point in the game requires it also works on spiritual damage and some esoteric effects.
And she just applied an 「Curse of Absolute Death」 on Shirou.
Gods, that is so bullshit.