Book of Shadows

Book of Shadows is an item used for spawning the boss Shadowlord.

Other methods
Using any Enchanted Book on an empowered Web Reaper will convert it into a Book of Shadows.

Spawning Shadowlord
The Book of Shadows' only use is for spawning Shadowlord.

When used on a Shadow Altar, Shadowlord will spawn. This will consume the Book of Shadows. If spawned on Peaceful difficulty, Shadowlord will despawn immediately, so it is important to ensure that the world is not on Peaceful difficulty.

History
影之书