Cutting through a Space Marines head and helmet

Brother Folau turned his head.

The knife entered the gap between his gorget and his helmet, thrusting upwards through the palate, skewering his tongue to the top of his mouth and then continuing upwards, through the ethmoid sinus and the basal ganglia before exiting the brain through the primary motor cortex and emerging, for a flicker-brief moment, sharp as the tongue of a chameleon, out through the top of his helmet before being withdrawn.

Brother Folau stood dumbly staring at her, all power of movement suddenly withdrawn from him by the precision of her strike. Then she drew her finger down over his helmet before, lightly, pressing. The Carcharodon, only now realising he was dead, toppled like a felled tree."