Book of Koridai

"Such is the power of the Prince of Darkness that he can kill with a single look. Attacks against Ganon will prove fruitless, unless Link attacks with the sacred book."

- Aypo

The Book of Kordiai is an essential item appearing in Link: The Faces of Evil. Link uses it to defeat Ganon in the final boss fight, as the book is capable of imprisoning evil.

In Zelda: The Wand of Gamelon, a similar item named the Wand of Gamelon is used to defeat Ganon.

History
Aypo mentions the Book of Koridai to Link, and offers to read its secret verse if he manages to retrieve it. Gameplay-wise, the secret verse is optional, as it simply reveals how to defeat Ganon. The Book of Koridai is locked away in the Shrine of Koridai, where it is guarded by the fourth boss, Glutko. Link obtains the item after the cyclops is defeated.

Later, in Ganon's Lair, Link can immediately defeat Ganon by throwing the Book of Koridai at him. It seals Ganon into one of its pages and sends him to "the pit".