They went under attack
As mentioned before, Cassie and the others were currently sailing on the vast open water using a simple-looking, yet self-operating wooden boat.
The place was relatively dark as though night had already fallen on the place.
But considering that they were in the Underworld right now where the concept of day and night did not seem to exist, time did not seem to be the reason as to why the place was dark.
As their boat continued to sail in the unknown waters, the deafening silence of the entire place started to get into Cassie's nerve.
And as though sensing His Highness' discomfort, Jace decided to break the silence.
"Your Highness, this is the River of Styx and the reason why the water in here has a deep color of violet was that its riverbed was made of pure amethyst."
Jace explained to Cassie since the other party's main purpose in tagging along with them was to learn a thing or two about the things that concerned the abodes of the undead.