Hades 2 is full of mythological items and tools for Melinoë to make use of while traveling through the Underworld, and one especially important tool is the Book of Shadows. Here's what you need to do to get it, and what you'll actually use it for on your journey to the underworld.
What is the Book of Shadows?
The Book of Shadows is an essential tool that gathers and compiles information about everything from slain enemies and bosses to unlocked weapons to Melinoë's relationship status with different characters (requires Incantation). Each time a new enemy is defeated, or when Melinoë progresses further in the story, more information is added to the Book of Shadows, providing a detailed insight into her journey and the world around her.