The blade appeared literally out of nowhere, and the moment Jacob realized that it was there, it was already touching his mask's eye, which was made with a basic-type-1 glass known as Sky Glass.
Sky glass has a clear reflection on one side while the other side looks like a solid surface, and it is quite sturdy and stainless. Not even an Advance Type-0 weapon could scratch its surface.
However, the assassins were clearly quite professional, and they probably knew what kind of target they were going to assassinate, so they clearly won't come with some type-0 weapons.
So, Jacob knew that even if he managed to evade, his eye was as good as gone. What he should be worried about was that blade managed to penetrate deep into his skull.
That's why Jacob started to retract his head backward while the blade edge was already breaking past the sky glass and now only a hair away from touching the cornea, and almost at the same time, it turned fiery crimson!
