The novel deals openly and often with subjects relating to trauma and mental health. Both Nico and Will work through their traumatic memories in the Underworld and tend to their mental wellness by examining their relationship to darkness. In the novel, “darkness” is a mutable signifier whose precise meaning is contextually dependent. At times it refers to negative emotions like sadness, pain, or jealousy. Other times it refers to mental health, which is “our emotional, psychological, and social well-being” (“About Mental Health.” Centers for Disease Control and Prevention, 2023). The book also sometimes refers to Nico’s specific mental health conditions, like PTSD, which in his case is related to traumas he experienced.

Readers should thus take care to closely examine a passage before assuming the word darkness refers to a specific mental health condition. For instance, when Will talks about his darkness, he is not referencing mental health, even though he does have painful memories. As the camp healer, he knows how “heartbreakingly mortal” (101) his friends are after seeing them dead and dying after battles featured throughout the Camp Half-Blood Chronicles. As he grew up, his mother was constantly touring, so he is familiar with isolation.