Book of Koridai

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 somehow capable of imprisoning evil.

Similarly, in Zelda: The Wand of Gamelon, the Wand of Gamelon item is used to defeat Ganon.

History
Earlier in the game, Aypo mentions the book to Link. He offers to read the Book of Koridai's secret verse if Link manages to retrieve it. Gameplay-wise, the secret verse is optional, as it simply reveals on 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 from defeating Glutko. Later, in the final boss fight, Link can easily defeat Ganon by throwing the Book of Koridai at him.