In the fourth volume of 白銀のソードブレイカー I found these sentences:
I thought it could mean something like しないと, I found this answer that seems to confirm my guess, and Jisho has an entry about this too, but with that meaning I'm not sure how should I understand those sentences.
I tried looking for it in my grammars, but I didn't find it, and a Google search didn't help solving my doubt.
If it's a negative form of する, I guess those sentences means something like:
"The Demon Sword was thrust into Durandaru's back, and without cutting her in two the blade approached", which I don't really understand: if the sword is already in the character's back (突き立てる), what does it mean that's approaching? If instead it's in the process of stabbing her and has not reached her too, why "without cutting her in two"? If the sword hasn't reached her yet, it's obvious she isn't yet cut in two.
"The light released by the blade wrapped her body, without controlling everything, it just overflown, everything slowed down, completely becoming dull, the Holy Sword approached Durandaru, without doubt this would hit...", which again I don't really understand: what should it mean that the light doesn't control everything?
Moreover, even reading them that way I'm not sure why せんと (with the と): it doesn't look like a quotative と, nor does it seem a conditional one, so I'm not really sure about how this structure is formed.