You will meet many gods in The Son of Neptune, but here's a fellow most mortals would prefer not to meet: Thanatos, the god of death. What part will he play, and what's up with those chains? For that, you will have to wait for the book. However, if you'd like to read up on Thanatos, I'd recommend one of my favorite online resources for Greek mythology: theoi.com.
Two more weeks until The Son of Neptune, and stay tuned for one more character reveal next Tuesday!