Her eyes widened a little when he made that promise, that most precious oath. Erica had be raised to believe the very word Styx was sacred and not to be misused, that if she swore something on Styx it had to be upheld even by the gods.
When she returned to America and spoke to him on the phone about having the chance to see the girls again she had made her own oath to him on Styx right then: that if he hurt any of them, she would find a way to destroy him even if it meant forfeiting her own life and soul.
But now here Hermes was making his oath and she knew there was no way he would risk going back on it.
For a few long moments she just watched him, searching his eyes for... something. She didn't know what she was looking for.