[He's gotten a lot of different responses in the past. People becoming genuinely frightened and fleeing. Heroes up to no good trying to feed him some lie. Grieving loved ones pleading. But he's never encountered this. Someone who genuinely didn't know of the River. For that matter, most people this close to his land recognize him, as well. Give some reaction of hatred or fear. So he has to blink for a moment.]

The- The River Styx. The border of the underworld? Of my domain.
[Like, come on. Surely she has to know. And she's just confused. He aught to scare her a bit.]

If you choose to enter my River, I'll have no choice but to keep you among the dead for the rest of time.