From Dune by Frank Herbert:
"I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain."
He felt calmness return, said: "Get on with it, old woman."
"Old woman!" she snapped. "You've courage, and that can't be denied. Well, we shall see, sirra." She bent close, lowered her voice almost to a whisper. "You will feel pain in this hand within the box. Pain. But! Withdraw the hand and I'll touch your neck with my gom jabbar --the death so swift it's like the fall of the headsman's axe. Withdraw your hand and the gom jabbar takes you. Understand?"
"What's in the box?"
Is the sentence "But!" considered grammatically correct? How is this meant to be interpreted? How is it different from using a comma? It should be noted this is part of a dialog.